feat: add lc 46, 39, 78

feat(work): add solution for LeetCode 323 number of connected components problem
feat: add lc 684
2026-11-02 13:12:44 +00:00 · 2026-09-24 14:36:19 -04:00 · 2026-09-23 15:40:39 -04:00 · 2026-09-23 15:22:26 -04:00 · 2026-09-22 15:10:30 -04:00 · 2026-09-22 15:02:31 -04:00
188 changed files with 9325 additions and 1093 deletions
@@ -114,7 +114,12 @@ One issue per curriculum topic; owns the problem sub-issues.
  ```

 `Scheduled` should come from the calendar of record — `apps/api/data/schedule.json`
-(day → topic issue) — not guessed.
+(`date → { topics: [issue, …] }`; a doubled day lists two) — not guessed.
+
+Gate problems (`gate: [...]` in the same file) must exist in the catalog before their
+Saturday, or the Worker refuses to create the review issue: create each as a
+`set:optional` problem sub-issue of its natural topic. Being named in the plan keeps it
+out of the drill pool, so it stays unseen until the gate.

 ## Reference tables

@@ -0,0 +1,150 @@
+---
+name: problem-approach
+description: Write the `## Approach` section — a Mermaid flowchart that traces the submitted solution — for every docs solution page under apps/docs/content/ that lacks one, using the repo's exact placement and node conventions. Use when asked to add/implement/fill in missing approaches, draw the approach diagram for a problem page, or when `/problem-approach` is run.
+---
+
+# Problem approach diagrams
+
+Every solution page (`apps/docs/content/(<category>)/<num>-<slug>.mdx`) has three
+hand-owned sections around the script-owned one:
+
+```
+frontmatter → <Badge> → ::::warning (maybe) → ### Example N / ### Constraints
+## Approach        ← this skill (Mermaid flowchart of the code below)
+## Solution        ← bun run sync owns this; never edit inside it
+## Explanation     ← bun run videos --write (NeetCode embed); not this skill
+```
+
+`sync.ts` regenerates only `## Solution`, so an `## Approach` placed **before** it
+survives every re-sync; anything placed inside `## Solution` is overwritten on the
+next `work/` push.
+
+## The command: fill every missing approach
+
+1. **Find the pages** (numbered pages with no `## Approach` heading):
+
+   ```sh
+   grep -L '^## Approach' apps/docs/content/*/*.mdx
+   ```
+
+2. **For each page**, read `## Solution` and write the diagram from *that code* — the
+   one on the page, not the textbook algorithm. The reader compares diagram and code
+   line by line; the diagram is a trace, not a lecture.
+
+3. **Insert** the section between the `### Constraints:` list and `## Solution`,
+   blank-line separated on both sides:
+
+   ````md
+   - `-100 <= Node.val <= 100`
+
+   ## Approach
+
+   ```mermaid
+   flowchart TD
+     …
+   ```
+
+   ## Solution
+   ````
+
+4. **Check** each diagram by hand against the syntax rules below (the docs build does
+   not parse Mermaid — a broken diagram fails only in the browser), then run the
+   docs build once at the end and review the pages in `bun run dev`:
+
+   ```sh
+   cd apps/docs && bunx blume build --isolated && rm -rf .blume-verify
+   ```
+
+Pages with two languages (`<CodeGroup>`) get **one** diagram, traced from the
+Python version. Pages with only JavaScript trace the JavaScript.
+
+## Diagram conventions
+
+Reference page: `apps/docs/content/(array)/15-3sum.mdx`. Every existing diagram
+follows this shape:
+
+```mermaid
+flowchart TD
+  S(["threeSum(nums)"]) --> O["nums.sort() — duplicates become adjacent"]
+  O --> F{"more i in 0..n-1?"}
+  F -- no --> E(["return result"])
+  F -- yes --> D{"nums[i] == nums[i-1]?"}
+  D -- yes --> F
+  D -- no --> P["left = i+1, right = n-1, target = -nums[i]"]
+  P --> W{"left < right?"}
+  W -- no --> F
+  W -- yes --> C["current = nums[left] + nums[right]"]
+  C --> Q{"current vs target"}
+  Q -- equal --> A["append triplet, skip equal neighbours, left += 1, right -= 1"]
+  A --> W
+  Q -- "current < target" --> L["left += 1 — need a bigger sum"]
+  L --> W
+  Q -- "current > target" --> R["right -= 1 — need a smaller sum"]
+  R --> W
+```
+
+- `flowchart TD`, two-space indent, one edge per line, edges listed in execution
+  order (entry first, exits as soon as they are reachable).
+- **Entry** is a stadium holding the signature: `S(["threeSum(nums)"])`. **Every
+  exit** is a stadium: `E(["return result"])`, `R(["return true"])`.
+- **Statements** are rectangles `["…"]`, in the code's own identifiers and operators
+  (`left += 1`, `freq[n] = (freq[n] or 0) + 1`). Several trivial assignments may share
+  one node, comma-separated.
+- **Decisions** are diamonds `{"…?"}` with `-- yes -->` / `-- no -->` edges. A
+  three-way branch names the outcomes: `-- equal -->`, `-- "current < target" -->`
+  (quote an edge label whenever it holds `<`, `>`, or spaces around operators).
+- **Loops** are a decision: `for` → `{"more n in nums?"}`, `while` → the condition
+  verbatim `{"left < right?"}`. `-- no -->` goes to whatever follows the loop; the
+  body's last node edges back to the decision. `continue` is an edge back to the loop
+  decision (`D -- yes --> F` above); `break` is an edge to the node after the loop.
+- **Recursion** is a statement, not an expanded subgraph: `A["left = invertTree(root.left)"]`.
+  The base case is the first decision after entry.
+- **Classes** (design problems) use one `subgraph name["method(args)"]` per public
+  method, plus a `state["…"]` subgraph describing the fields, with `state --> method`
+  edges — see `(hash-table)/146-lru-cache.mdx`. Private helpers are named inside the
+  method nodes (`"_remove_node then _add_node — move it beside the tail"`), not drawn.
+- **Annotate the why with an em dash** inside the node, sparingly (one clause, a few
+  per diagram): `"nums.sort() — duplicates become adjacent"`, `"map = {0: 1} — the
+  empty prefix"`. The dash explains the invariant that makes the step correct — the
+  one thing the code does not say.
+- **Node ids**: one or two capitals, unique per diagram (`S I L Q C E R W A B`). Class
+  diagrams prefix per method (`G1`, `P1`).
+
+### Syntax rules (a diagram that breaks the page is worse than none)
+
+- Wrap **every** label in double quotes — brackets, parentheses, `<`, `>` and `|` in an
+  unquoted label break the parser.
+- Never put `"` inside a label; use `'` for string literals (`"s == ''?"`).
+- Never use `|` anywhere — not in node labels, not as `-->|text|` edge syntax; edge
+  labels are always `-- text -->`. Write `or` for a logical or.
+- Every id an edge references must be declared with a shape the first time it appears.
+- Balanced: each `subgraph` has an `end`; each fence opens ```` ```mermaid ```` and closes ```` ``` ````.
+
+### Code that does not run
+
+Trace it anyway — the diagram documents what was submitted — and put a `::::warning`
+callout **before** the fence naming exactly why it fails (`(array)/121-best-time-to-buy-and-sell-stock.mdx`):
+
+````md
+## Approach
+
+::::warning
+The diagram traces the code below literally. That code does not run: `curr` and `ans` are never initialised, so the first `curr > left` test raises `NameError`.
+::::
+
+```mermaid
+…
+```
+````
+
+Do not fix the code on the page: `## Solution` mirrors `work/1`, and the fix belongs
+there (then `bun run sync`).
+
+## Do not
+
+- Do not write prose approaches, complexity tables or alternative algorithms — the
+  section is one flowchart (plus the warning callout when the code is broken).
+- Do not touch `## Solution` or anything after it; `## Explanation` is filled by
+  `bun run videos --write`.
+- Do not add an approach to `reference/` pages or `index.mdx` — only numbered
+  solution pages (`title: 'NN. …'`) carry one.
@@ -0,0 +1,35 @@
+#!/bin/sh
+# Pre-commit: the docs pages under apps/docs/content are generated from work/1
+# by `bun run sync`, and deploy.yml publishes whatever main holds. Regenerate
+# them now and stage what sync changed, so a solution and its page land in the
+# same commit and a stale `## Solution` never reaches main.
+#
+# Sync reads the working tree, not the index, so the pages it writes only
+# match this commit when work/1 has nothing unstaged: an untracked or
+# half-staged solution would ship a page whose source is not in the commit.
+# In that case refuse and say what to stage.
+#
+# Only paths that became dirty during sync are staged. Anything already dirty
+# beforehand (hand-written `## Approach` prose in progress, a new index.mdx)
+# stays where the author left it.
+#
+# Bypass for a deliberate partial commit: git commit --no-verify
+set -eu
+
+cd "$(git rev-parse --show-toplevel)"
+
+unstaged=$(git status --porcelain -- work/1 | grep -v '^[MADR] ' || true)
+if [ -n "$unstaged" ]; then
+  printf 'pre-commit: work/1 has unstaged changes; stage or stash them so the generated pages match this commit:\n%s\n' "$unstaged" >&2
+  exit 1
+fi
+
+before=$(mktemp)
+trap 'rm -f "$before"' EXIT
+git status --porcelain -- apps/docs/content | sort > "$before"
+report=$(bun run --silent sync)
+generated=$(git status --porcelain -- apps/docs/content | sort | comm -13 "$before" - | cut -c4-)
+if [ -n "$generated" ]; then
+  printf '%s\n' "$report" | grep -v 'skipped (in sync)' >&2
+  printf '%s\n' "$generated" | xargs git add --
+fi
@@ -23,6 +23,7 @@ flowchart LR
  D1 -->|Sat midnight ET| Review[Review — Week N issue]
  PI -->|"spaced-repetition.yml (6 AM ET cron, closed +3d/+7d)"| SR[Spaced Repetition — Month D issue]
  D1 --> Charts[SVG charts + /api/stats]
+  Charts -->|bun run stats| TUI[terminal dashboard]
 ```

 - **Reconcile, don't react.** `close-solved.ts`, `close-topics.ts`, and the Worker's catalog sync recompute desired state from scratch each run: re-runs are no-ops, backfills need no special casing, closing is one-directional. `close-solved` posts its *whole* implemented set to `/admin/solved`, not just the issues it closed this run.
@@ -32,6 +33,7 @@ flowchart LR
 - **The picker owns the local review loop; the buckets are its state.** `work/` holds three buckets (`1` = first solve, `3`/`7` = blind re-solves, named after the ladder rungs) and `bun run pick` has three sections — Tab cycles them. Pools are reconciled from the filesystem plus one `git log` (`apps/cli/work.ts`): a first solve dates from the ET date of the commit that added its `work/1` file — the same push that closes the problem issue and starts the +3/+7 windows — and a window stops being owed the moment its bucket holds a file. Uncapped, unlike the review issue's `WINDOW_CAP`: that issue is one day's assignment, the picker is everything still owed. An empty scaffold under `work/1` owes nothing (it is unsolved), which is why `isImplemented()` lives in `apps/cli/source.ts` and is shared with `close-solved.ts`. leetcode-cli reads its output directory from `~/.leetcode/workspaces/<active>/config.json` and nowhere else — no flag, no env var — so `pick.ts` MERGES `workDir` into that file around the child (never replays saved bytes: the CLI owns the file too) and leaves it pointed at `work/1`, then treats the scaffold's existence as the acceptance test, since leetcode-cli exits 0 even when an expired session made it write nothing. The picker itself writes no state: pushing the re-solve is what moves the ladder, via `close-solved` → `/admin/solved` with that file's bucket.
 - **The README charts are live, never committed.** They are Worker endpoints reading D1 per request (`Cache-Control: max-age=300`, honored by GitHub Camo), so a solved push moves them within ~5 minutes with no commit and no workflow of their own.
 - **D1 owns SRS state; GitHub issues own the catalog; `apps/api/data/schedule.json` owns the calendar.** The catalog reconcile never invents rows and never overwrites SRS columns (`stage`, `next_review`). `Target Date` is the *only* Project field the Worker mirrors (`apps/api/src/mirror.ts`): failures are warnings, never lost D1 writes; topic rows' `Target Date` is never written. SRS stage and first-attempt result live **only** in D1 — do not re-add them as Project fields. The Project's `Set`/`Difficulty` single-selects are a projection of the issue labels, reconciled by `bun run sync-project-fields` — a best-effort final step of `close-solved.yml`, never the Worker.
+- **The plan names days; the samplers fill the gaps.** `apps/api/data/schedule.json` maps a date to a `ScheduledDay` (`apps/api/src/srs.ts`): `topics` (one or more topic issues — a doubled day lists two, and the digest renders each with its core list then the optional "if time" list), `problems` (a set day: fixed numbers, rendered like a topic), `gate` (Saturday's blind set, verbatim — `createReviewIssue` refuses to create the issue if any number is not a catalog row, so gate problems must exist as `set:optional` sub-issues of their topic first), or `sweep` (every problem whose last attempt failed, taps to re-log, no drills). Every number a `gate` or `problems` entry names is in `RESERVED`, and the drill/gate samplers never draw it — that is what keeps a planned gate problem unseen until its Saturday. Dates absent from the plan still get the ladder's reviews and drills; a Saturday without a `gate` entry falls back to sampling this week's optional pools.
 - **Determinism = idempotency.** Drill/gate sampling uses a seeded PRNG (`rng()` in `apps/api/src/srs.ts`, FNV-1a → mulberry32, seed = date / ISO week); the digest is keyed by ET date in `email_log`; one-tap links are unique on (problem, date, kind).
 - **DST-proof crons:** each event has two UTC crons; code fires only on the computed ET hour (`etHour`), unit-checked in `apps/api/src/srs.test.ts`.
 - **Chained workflows:** `GITHUB_TOKEN`-driven issue closes fire no `issues` events, so `close-topics.yml` chains off `workflow_run` of Close Solved instead. The Worker's webhook uses its own `GH_PAT`, so its events flow normally.
@@ -42,9 +44,10 @@ flowchart LR
 |---|---|
 | `work/<1\|3\|7>/<Difficulty>/<Category>/<num>.<slug>.{js,py}` | Solutions, e.g. `work/1/Easy/Array/1.two-sum.py`. The top directory is the spaced-repetition bucket: `1` = first solve, `3`/`7` = the blind re-solves of that problem 3 and 7 days later. Machine-parsed header comment (title / `Difficulty:` / URL / `─` rule / statement) — preserve its exact shape |
 | `apps/cli/` | Flat Bun TS workspace: automation entries (shebang + top-level await) and libraries (no shebang, side-effect-free on import). Root `bun run` scripts delegate here |
-| `apps/docs/` | Blume site (`blume.config.ts`, `content/`, `islands/`, `public/`). `content/(<category>)/<num>-<slug>.mdx` generated by sync |
-| `apps/api/` | Cloudflare Worker workspace: `src/` modules, `data/schedule.json` (day → topic issue, human-edited, bundled at deploy), `migrations/` (append-only; `0002` is the +3/+7 ladder rebuild) |
+| `apps/docs/` | Blume site (`blume.config.ts`, `content/`, `components/` + `components.ts`, `islands/`, `public/`). `content/<topic>/<num>-<slug>.mdx` plus one `meta.ts` per topic folder, all generated by sync: the folder is the README campaign topic slug (`tree-dfs/`, `heap-priority-queue/`) and a real route segment (`/tree-dfs/invert-binary-tree`), the `meta.ts` carries the exact label and topic number so the sidebar reads in curriculum order, and the page badge is the `work/` data-structure category. A hand-written `<topic>/index.mdx` (`hidden: true`) is that topic's landing page: Blume ≥ 1.7.3 links the sidebar group row to it (`sliding-window/index.mdx` → `/sliding-window`) and the hidden flag drops the duplicate row. Two kinds of custom MDX component, both import-free on every page: `islands/<Name>.tsx` is hydrated React (auto-discovered; `ProgressDashboard`, `DependencySpine` — the README topic map as data, zoom/pan, nodes link to whatever route Blume's `/api/docs/navigation.json` reports for the topic folder, inert until a topic has pages), `components/<Name>.astro` registered in `components.ts` is static markup plus a custom element for behaviour, the way Blume's own `Frame` is built (`Carousel`/`CarouselImage`: `src` takes a URL, `/file.png`, or the page-relative `../../public/file.png`) |
+| `apps/api/` | Cloudflare Worker workspace: `src/` modules, `data/schedule.json` (the plan: `date → { topics | problems | gate | sweep }`, human-edited, bundled at deploy), `migrations/` (append-only; `0002` is the +3/+7 ladder rebuild) |
 | `.github/workflows/` | deploy (docs → Pages, Worker → Cloudflare, on every main push), close-solved (issues + `/admin/solved` + Project `Set`/`Difficulty`, needs `SRS_ADMIN_KEY` and `PROJECT_PAT`), close-topics, sync-d1 (issue edits → `/admin/reconcile`), spaced-repetition (daily 06:00 ET cron → today's review issue; `GITHUB_TOKEN` only, no secrets) |
+| `.githooks/` | Tracked git hooks; `bun install` points `core.hooksPath` here (root `prepare` script, no husky). `pre-commit` runs `bun run sync` and stages only the pages sync changed, so a solution and its page land in one commit; it refuses to run while `work/1` has anything unstaged (sync reads the tree, so the page would not match the commit), and leaves pre-existing dirt under `apps/docs/content` (in-progress `## Approach` prose) unstaged. `git commit --no-verify` for a deliberate partial commit |

 ## Development Commands

@@ -56,11 +59,18 @@ bun run pick            # scaffold a solution via leetcode-cli (fuzzy picker; Ta
                        # re-solves → work/3, work/7. Each section hides what its own bucket
                        # already holds; index+descriptions cached in
                        # ~/.local/share/leetcode/data.db — see apps/cli/db.ts)
+bun run stats           # terminal dashboard over GET /api/stats — four panes (Overview,
+                        # Ladder, Concepts, Activity); Tab/1-4 switches, ↑↓ scrolls,
+                        # r refetches, q quits. Outside a TTY it prints every pane once.
+                        # SRS_API points it at a local Worker like close-solved does.
 bun run test            # run ONE solution against LeetCode's judge (not a test suite;
                        # most recently tested first, stamped in the db on exit 0)
 bun run submit          # submit ONE solution to LeetCode (newest scaffold first, nothing
                        # hidden; last_submitted stamped in the db on exit 0)
 bun run sync            # work/1 → apps/docs/content/ pages (re-solves are never published)
+bun run videos          # docs pages with no <YouTube> embed → NeetCode video per page
+                        # (channel search scraped, no API key; --write appends the
+                        # `## Explanation` embed; "no NeetCode match" pages stay untouched)
 bun run dev|build       # Blume docs site (runs in apps/docs/)
 bun run close-solved -- --dry-run    # issue reconcilers (also DRY_RUN=1)
 bun run close-topics -- --dry-run
@@ -92,12 +102,13 @@ curl -X POST -H "Authorization: Bearer $LINK_KEY" \

 ## Important Files

- `apps/api/src/srs.ts` — SRS domain: the ladder (`new → +3 → +7 → retired`; fail resets to `+3`; first-ever log enters at `+3`), `WINDOWS`/`STAGES` (the one vocabulary shared by the charts, the `work/<n>` buckets and the review issues), ET date math, `workingDay()` (no scheduled date is ever a Sunday), seeded sampling, `logAttempt()` (the ONE write path — email taps, webhook, gate scoring all converge here), and overload leveling: at most `REVIEW_CAP` (3) reviews surface per day — `levelReviews()` (run by `sendDigest` on real sends only, never dry) gives everything past the cap a concrete future WORKING day, ≤ 3 per day, oldest first, instead of letting the due pile grow.
+- `apps/api/src/srs.ts` — SRS domain: the ladder (`new → +3 → +7 → retired`; fail resets to `+3`; first-ever log enters at `+3`), `WINDOWS`/`STAGES`/`TEMPERATURES`/`DIFFICULTIES`/`SETS` (the one vocabulary shared by the charts, the `work/<n>` buckets, the review issues and the stats dashboard), ET date math, `workingDay()` (no scheduled date is ever a Sunday), `temperatureOf()` (days since a concept was last exercised → `hot`/`fresh`/`fading`/`cold`, bands derived from `WINDOWS` so they cannot drift; read-side only, nothing is scheduled off it), seeded sampling, `logAttempt()` (the ONE write path — email taps, webhook, gate scoring all converge here), and overload leveling: at most `REVIEW_CAP` (3) reviews surface per day — `levelReviews()` (run by `sendDigest` on real sends only, never dry) gives everything past the cap a concrete future WORKING day, ≤ 3 per day, oldest first, instead of letting the due pile grow.
 - `apps/api/src/index.ts` — router + cron dispatch; `apps/api/wrangler.jsonc` — bindings (`DB`, `EMAIL`), crons, vars; secrets `GH_PAT`/`WEBHOOK_SECRET`/`LINK_KEY` via `wrangler secret put`.
 - `apps/api/src/digest.ts` / `apps/api/src/email.tsx` — the daily digest, split data/presentation. `digest.ts` reads D1 into a `DigestData`; `email.tsx` owns every colour and every sentence, and renders both the HTML and (via its own `plainDigest`, not React Email's `plainText` mode, which flattens the tables) the text alternative. The retrieval rules hold by construction: `DigestRow` has no title and no issue field, so a review or drill line *cannot* leak the topic or a solution link. Subject is `(Day N/56) LeetCode Daily Digest`.
 - `apps/api/src/png.ts` — hand-rolled PNG encoder (RGB8, one IDAT, zlib via `CompressionStream("deflate")`, CRC32, 5×7 bitmap font). It exists because every major email client refuses remote SVG, so `/chart/heatmap.png` rasters the heatmap for the digest while `/chart/heatmap.svg` keeps serving the README byte-for-byte. Both come from one `heatmapCells()` so the two pictures cannot drift.
- `apps/cli/sync.ts` — work→docs contract: only `## Solution` onward is script-owned on existing pages; human prose is never touched; never hand-write solution pages or edit inside `## Solution`.
- `apps/cli/work.ts` — the `work/` layout: bucket paths, `firstSolved()` (one `git log`, ET dates, keyed by LC number so past layout moves don't matter), and `reviewQueues()` (what each window still owes). Everything that needs to know where a solution file lives, or when it was solved, goes through here.
+- `apps/cli/sync.ts` — work→docs contract: on existing pages only the `<Badge>` line and the `## Solution` section are script-owned, the latter bounded by `solutionEnd()` at the next unfenced top-level heading; human prose before it and any sections after it are never touched. Never hand-write solution pages or edit inside `## Solution`. Two classifications, two homes: the sidebar group is the README topic (`readmeTopics()`, first listing wins for a problem under two topics; a warm-up outside the plan groups under its category at `order: 0`, ahead of topic 1) and the badge is the `work/1/<Difficulty>/<Category>/` category — topic folders and their `meta.ts` are rewritten every run, a page is `rename`d when its topic changes, and a folder every page has left is deleted unless it still holds a hand-written `index.mdx` (sync never writes, moves, or deletes one). Never use `(group)` folders here: they add no route segment, so a topic `index.mdx` inside one collides with the root `/`. The two hand-owned sections around the solution have their own owners: `## Approach` (a Mermaid flowchart tracing the page's code, placed *before* `## Solution` so a re-sync keeps it) is written per the `problem-approach` skill (`/problem-approach` fills every page missing one); `## Explanation` (the NeetCode `<YouTube>` embed, after `## Solution`) is appended by `bun run videos --write` (`apps/cli/videos.ts`).
+- `apps/cli/work.ts` — the `work/` layout: bucket paths, `isSolution()`/`solvedInBucket()` (the stub test, shared with `close-solved.ts`), `firstSolved()` (one `git log`, ET dates, keyed by LC number so past layout moves don't matter), and `reviewQueues()` (what each window still owes). Everything that needs to know where a solution file lives, or when it was solved, goes through here.
+- `apps/api/src/stats.ts` / `apps/cli/stats.ts` / `apps/cli/tui.ts` / `apps/cli/api.ts` — the read side. `buildStats()` is the ONE aggregation: phases, ladder, difficulty, per-topic temperature, gates, the 14-day due queue, and `heat` (every campaign day, zeros and future days included — the docs page windows the tail off it rather than asking a second question). Two readers consume it, the docs island and `bun run stats`, so its shape is both their contract: change all three together. `tui.ts` is hand-rolled ANSI (bars, sparklines, heat cells, alternate screen) on charts.ts's palette, and every chart differs by GLYPH as well as by colour so a piped or `NO_COLOR` dashboard still reads. The TUI recomputes nothing: the only local fact it shows is the `work/` solution count, which is inventory, not schedule.
 - `README.md` live charts are Worker endpoints (`/chart/*.svg`, `/badge/gate.svg`, 5-min Camo cache); the docs `/progress` page fetches `/api/stats` client-side (`apps/docs/islands/ProgressDashboard.tsx`).
 - `apps/docs/blume.config.ts` — site base `/leetcode`; `README.md` campaign table doubles as topic-map input to `sync.ts` (`readmeTopics()`, `TOPIC_ALIASES`).
 - Old `.github/srs/` JSON state, its `srs-*` Actions, and the one-shot `import-srs.ts` importer are RETIRED and deleted — do not resurrect; git history is the record.
@@ -105,8 +116,7 @@ curl -X POST -H "Authorization: Bearer $LINK_KEY" \
 ## Runtime/Tooling Preferences

 - **Bun only**: run scripts with `bun apps/cli/<name>.ts` (or the root `bun run` aliases), install with `bun install --frozen-lockfile`. One workspace (`apps/*`), one root `bun.lock`. Node 22 appears only in `deploy.yml` because Blume requires it.
- **No root tsconfig, no ESLint, no Prettier** — match surrounding style by hand. `apps/api/` has a strict `tsconfig.json`; `Env` types are generated (`bunx wrangler types`), never hand-written.
- **Near-zero runtime npm dependencies** — SVG, PNG, HMAC (WebCrypto), GraphQL are hand-rolled; Actions scripts use Bun builtins + `fetch` only. The **one** exception is the digest email: `apps/api/src/email.tsx` renders React Email (`react`, `react-dom`, `@react-email/components`, `@react-email/render`) inside the Worker, because hand-rolling table-based email HTML that survives Gmail *and* Outlook is not worth owning. It costs ~235 KB gzipped of a 3 MB budget, needs no `nodejs_compat` (it resolves to `renderToReadableStream`), and requires `"jsx": "react-jsx"` in `apps/api/tsconfig.json`. Do not extend this exception to any other module.
+- **Near-zero runtime npm dependencies** — SVG, PNG, terminal ANSI, HMAC (WebCrypto), GraphQL are hand-rolled; Actions scripts use Bun builtins + `fetch` only. The **one** exception is the digest email: `apps/api/src/email.tsx` renders React Email (`react`, `react-dom`, `@react-email/components`, `@react-email/render`) inside the Worker, because hand-rolling table-based email HTML that survives Gmail *and* Outlook is not worth owning. It costs ~235 KB gzipped of a 3 MB budget, needs no `nodejs_compat` (it resolves to `renderToReadableStream`), and requires `"jsx": "react-jsx"` in `apps/api/tsconfig.json`. Do not extend this exception to any other module.
 - Local GitHub auth falls back to `gh auth token`; scripts run fine outside Actions.

 ## Testing & QA
@@ -4,8 +4,8 @@

 <p align="center">
  <a href="https://x.com/basedprad"><img alt="follow" src="https://shieldcn.dev/x/follow/basedprad.svg" /></a>
-  <a href="https://lc.prad.nu"><img alt="badge" src="https://shieldcn.dev/badge/Read-Documentation.svg?logo=lu%3ABook" /></a>
-  <a href="https://lc.prad.nu/progress"><img alt="gate pass rate" src="https://srs-api.prdlk.workers.dev/badge/gate.svg" /></a>
+  <a href="https://leetcode.com/progress"><img alt="Custom badge" src="https://shieldcn.dev/badge/view-@prdlk.svg?logo=leetcode" /></a>
+  <a href="https://prdlk.github.io/leetcode"><img alt="badge" src="https://shieldcn.dev/badge/Read-Documentation.svg?logo=lu%3ABook" /></a>
 </p>

 <p align="center">
@@ -337,4 +337,3 @@
    <tr><td align="center"><code>B</code></td><td align="center">252</td><td><a href="https://leetcode.com/problems/meeting-rooms/">Meeting Rooms I</a></td></tr>
  </tbody>
 </table>
-
@@ -14,8 +14,12 @@ Live at `https://srs-api.prdlk.workers.dev`.
  labels, milestones. `catalog.ts` reconciles them into D1 nightly (and via
  `/admin/reconcile`); it recomputes from scratch, never invents rows, and
  never overwrites SRS-owned columns (`stage`, `next_review`).
- **The repo owns the schedule** — `data/schedule.json`, human-edited,
-  bundled at deploy. Git history is its audit log.
+- **The repo owns the plan** — `data/schedule.json`, human-edited,
+  bundled at deploy. Git history is its audit log. Each date is a
+  `ScheduledDay`: `topics` (one or more topic issues), `problems` (a set day,
+  fixed numbers), `gate` (Saturday's blind set, verbatim), or `sweep` (redo
+  every miss, nothing new). Numbers a `gate` or `problems` entry names are
+  `RESERVED`: the drill and gate samplers never draw them.
 - The GitHub Project mirror writes `Target Date` and nothing else: it is
  best-effort, so failures log warnings and never block a D1 write, and topic
  rows are never written. SRS stage and first-attempt result stay in D1 only —
@@ -47,7 +51,10 @@ raw `Date` math.
 Blind drills draw only from topics **already learned**: scheduled in an
 earlier week (the current week's optional pool is reserved for Saturday's
 gate) and with at least one core problem on the ladder — a skipped learning
-day never feeds drills just because its calendar week lapsed.
+day never feeds drills just because its calendar week lapsed. A Saturday
+whose plan fixes the `gate` set uses that list as-is; every number must be a
+catalog row (a `set:optional` sub-issue of its topic), or the review issue is
+not created — a `/done` on an uncatalogued number could never be logged.

 ## Routes

@@ -1,26 +1,28 @@
 {
-  "2026-08-17": 3,
-  "2026-08-18": 4,
-  "2026-08-24": 5,
-  "2026-08-25": 6,
-  "2026-08-26": 7,
-  "2026-08-27": 8,
-  "2026-08-28": 9,
-  "2026-08-31": 10,
-  "2026-09-01": 11,
-  "2026-09-02": 12,
-  "2026-09-03": 13,
-  "2026-09-04": 14,
-  "2026-09-07": 15,
-  "2026-09-08": 16,
-  "2026-09-09": 17,
-  "2026-09-10": 18,
-  "2026-09-11": 19,
-  "2026-09-14": 20,
-  "2026-09-15": 21,
-  "2026-09-16": 22,
-  "2026-09-17": 23,
-  "2026-09-18": 24,
-  "2026-09-21": 25,
-  "2026-09-22": 26
+  "2026-08-17": { "topics": [3] },
+  "2026-08-18": { "topics": [4] },
+  "2026-08-24": { "topics": [5] },
+  "2026-08-25": { "topics": [6] },
+  "2026-08-26": { "topics": [7] },
+  "2026-08-27": { "topics": [8] },
+  "2026-08-28": { "topics": [9] },
+  "2026-08-31": { "topics": [10] },
+  "2026-09-01": { "topics": [11] },
+  "2026-09-02": { "topics": [12] },
+  "2026-09-03": { "topics": [13] },
+  "2026-09-04": { "topics": [14] },
+  "2026-09-07": { "topics": [15] },
+  "2026-09-08": { "topics": [16] },
+  "2026-09-21": { "topics": [17, 18] },
+  "2026-09-22": { "topics": [19, 20] },
+  "2026-09-23": { "topics": [21] },
+  "2026-09-24": { "topics": [22] },
+  "2026-09-25": { "topics": [23] },
+  "2026-09-26": { "gate": [648, 1091, 802, 547, 90] },
+  "2026-09-28": { "topics": [24] },
+  "2026-09-29": { "topics": [25] },
+  "2026-09-30": { "topics": [26] },
+  "2026-10-01": { "problems": [399, 261, 909, 77, 52] },
+  "2026-10-02": { "sweep": true },
+  "2026-10-03": { "gate": [347, 1254, 1971, 131, 494, 986] }
 }
@@ -10,14 +10,11 @@
 * so relabelling a problem is enough to move it and titles stay readable.
 * The `problem` label is what marks a sub-issue as curriculum.
 */
-import { workingDay } from "./srs.ts";
+import { DIFFICULTIES, SETS, workingDay } from "./srs.ts";
 import type { GitHub } from "./github.ts";

 const TITLE_RE = /^LC (\d+) · (.+)$/;

-const DIFFICULTIES = ["easy", "medium", "hard"] as const;
-const SETS = ["core", "optional", "deferred"] as const;
-
 /** Deferred Hards enter the queue from this date, two per working day. */
 const DEFER_FROM = "2026-09-28";

@@ -5,14 +5,16 @@
 * the HTML and the plain-text alternative from that single tree (React Email
 * `render`), so the two can never drift.
 *
- * Retrieval rules: review and drill rows carry number + difficulty only —
- * never the topic, never a solution link. `DigestRow` has no field for
- * either, so the rule holds by construction. The learning day's core list is
- * the only labeled section. At most REVIEW_CAP (3) reviews per day; drills
- * fill toward DAILY_CAP (6) only when the due queue is light. Overflow is
- * levelled, not stacked: sendDigest gives every review past the cap a
- * concrete future date, ≤ REVIEW_CAP per day, oldest first. Sunday is a
- * two-line rest note.
+ * Retrieval rules: review, drill and sweep rows carry number + difficulty
+ * only — never the topic, never a solution link. `DigestRow` has no field for
+ * either, so the rule holds by construction. The plan's own sections — a
+ * learning day's topics (core, then the optional "if time" list) and a set
+ * day's problems — are the only labeled, linked ones. At most REVIEW_CAP (3)
+ * reviews per day; drills fill toward DAILY_CAP (6) only when the due queue
+ * is light, and never on a sweep day, which assigns every open miss and
+ * nothing new. Overflow is levelled, not stacked: sendDigest gives every
+ * review past the cap a concrete future date, ≤ REVIEW_CAP per day, oldest
+ * first. Sunday is a two-line rest note.
 *
 * Idempotency: email_log keys sends by ET date — a same-day re-send is a
 * no-op unless forced; the body itself is deterministic (drill picks are
@@ -22,20 +24,23 @@
 import {
  CAMPAIGN_DAYS,
  type ProblemRow,
+  RESERVED,
  REVIEW_CAP,
  SCHEDULE,
+  type ScheduledDay,
  addDays,
  campaignDay,
  campaignWeek,
  dueReviews,
  isoWeek,
  levelReviews,
+  lookupProblems,
  pickDrills,
  prettyDate,
  streak,
  weekdayOf,
 } from "./srs.ts";
-import { type DigestData, type DigestRow, renderDigest } from "./email.tsx";
+import { type DigestData, type DigestRow, type TopicRow, renderDigest } from "./email.tsx";
 import { signLink } from "./links.ts";

 /** Total daily ceiling: drills only fill what the whole due queue leaves. */
@@ -63,23 +68,50 @@ async function tapRow(env: Env, p: ProblemRow, date: string, staged: boolean): P
  };
 }

+/** A plan-named problem: linked to its issue, ticked once it has entered the ladder. */
+function topicRow(env: Env, p: ProblemRow): TopicRow {
+  return {
+    lc: p.lc_number,
+    difficulty: p.difficulty,
+    url: `https://github.com/${env.REPO}/issues/${p.issue}`,
+    solved: p.stage !== "new",
+  };
+}
+
+/** Problems whose most recent attempt failed — the miss log a sweep day works through. */
+async function openMisses(db: D1Database): Promise<ProblemRow[]> {
+  const { results } = await db
+    .prepare(
+      `SELECT p.* FROM problems p
+       JOIN attempts a ON a.lc_number = p.lc_number
+       WHERE a.id = (SELECT MAX(id) FROM attempts WHERE lc_number = p.lc_number)
+         AND a.result = 'fail' AND p.stage != 'retired'
+       ORDER BY a.date, p.lc_number`,
+    )
+    .all<ProblemRow>();
+  return results;
+}
+
 /** Everything the email needs, read straight out of D1 and the schedule. */
 export async function collectDigest(env: Env, date: string): Promise<DigestData> {
  const db = env.DB;
  const rest = weekdayOf(date) === 0;
+  const plan: ScheduledDay = rest ? {} : (SCHEDULE[date] ?? {});

  // Reviews surface at most REVIEW_CAP; drills fill toward DAILY_CAP only
  // when the WHOLE due queue is light — a heavy backlog must never trade
-  // review slots for brand-new drill problems.
+  // review slots for brand-new drill problems — and never on a sweep day.
  const due = rest ? [] : await dueReviews(db, date);
  const capped = due.slice(0, REVIEW_CAP);
-  const drills = rest ? [] : await pickDrills(db, date, DAILY_CAP - Math.min(due.length, DAILY_CAP));
+  const drills =
+    rest || plan.sweep ? [] : await pickDrills(db, date, DAILY_CAP - Math.min(due.length, DAILY_CAP));

  const data: DigestData = {
    day: prettyDate(date),
    progress: `Day ${campaignDay(date)} of ${CAMPAIGN_DAYS} · Week ${campaignWeek(date)}`,
    streak: await streak(db, date),
    rest,
+    topics: [],
    reviews: await Promise.all(capped.map((p) => tapRow(env, p, date, true))),
    carried: due.length - capped.length,
    drills: await Promise.all(drills.map((p) => tapRow(env, p, date, false))),
@@ -88,28 +120,42 @@ export async function collectDigest(env: Env, date: string): Promise<DigestData>
    progressUrl: `${env.DOCS_URL}/progress`,
  };

-  // New topic — the only section allowed to name problems and link them.
-  const topicIssue = rest ? undefined : SCHEDULE[date];
-  if (topicIssue !== undefined) {
+  // New topics — the plan's sections, the only ones allowed to name problems
+  // and link them. Core first; the optional set is "if time", minus anything
+  // the plan has reserved for a gate or a set day.
+  for (const topicIssue of plan.topics ?? []) {
    const topic = await db
      .prepare("SELECT name FROM topics WHERE issue = ?")
      .bind(topicIssue)
      .first<{ name: string }>();
-    const { results: core } = await db
+    const { results } = await db
      .prepare(
-        "SELECT * FROM problems WHERE topic_issue = ? AND set_label = 'core' ORDER BY lc_number",
+        "SELECT * FROM problems WHERE topic_issue = ? AND set_label IN ('core', 'optional') ORDER BY lc_number",
      )
      .bind(topicIssue)
      .all<ProblemRow>();
-    data.topic = {
+    data.topics.push({
      name: topic?.name ?? `#${topicIssue}`,
-      core: core.map((p) => ({
-        lc: p.lc_number,
-        difficulty: p.difficulty,
-        url: `https://github.com/${env.REPO}/issues/${p.issue}`,
-        solved: p.stage !== "new",
-      })),
-    };
+      core: results.filter((p) => p.set_label === "core").map((p) => topicRow(env, p)),
+      ifTime: results
+        .filter((p) => p.set_label === "optional" && !RESERVED.has(p.lc_number))
+        .map((p) => topicRow(env, p)),
+    });
+  }
+
+  // A set day assigns problems by number rather than by topic.
+  if (plan.problems) {
+    const { rows, missing } = await lookupProblems(db, plan.problems);
+    if (missing.length) console.warn(`digest ${date}: planned problems not in the catalog: ${missing.join(", ")}`);
+    data.assigned = rows.map((p) => topicRow(env, p));
+  }
+
+  // A sweep day works the miss log: every problem still standing on a
+  // failure, minus the ones today's review list already carries.
+  if (plan.sweep) {
+    const reviewed = new Set(capped.map((p) => p.lc_number));
+    const misses = (await openMisses(db)).filter((p) => !reviewed.has(p.lc_number));
+    data.sweep = await Promise.all(misses.map((p) => tapRow(env, p, date, true)));
  }

  // Saturday: the review issue already exists (created at midnight ET).
@@ -83,7 +83,7 @@ export interface DigestRow {
  failUrl: string;
 }

-/** A learning-day core problem — the only rows allowed a link. */
+/** A plan-named problem — the only rows allowed a link. */
 export interface TopicRow {
  lc: number;
  difficulty: string;
@@ -91,6 +91,13 @@ export interface TopicRow {
  solved: boolean;
 }

+/** One learning-day topic: the core list, then the optional "if time" list. */
+export interface TopicSection {
+  name: string;
+  core: TopicRow[];
+  ifTime: TopicRow[];
+}
+
 export interface DigestData {
  /** "Tuesday, August 25" */
  day: string;
@@ -98,11 +105,16 @@ export interface DigestData {
  progress: string;
  streak: number;
  rest: boolean;
-  topic?: { name: string; core: TopicRow[] };
+  /** Topics taught today, in plan order — empty on every other day. */
+  topics: TopicSection[];
+  /** A set day's problems, assigned by number rather than by topic. */
+  assigned?: TopicRow[];
  reviews: DigestRow[];
  /** Reviews past the daily cap; levelled onto the coming days. */
  carried: number;
  drills: DigestRow[];
+  /** A sweep day's miss log: present (even empty) only on a sweep day. */
+  sweep?: DigestRow[];
  gate?: { week: number; url: string };
  yesterday: { total: number; failed: number[] };
  gateRate?: number;
@@ -278,10 +290,15 @@ function Block({
 // ── email ────────────────────────────────────────────────────────

 function DigestEmail({ data }: { data: DigestData }) {
-  const load = data.reviews.length + data.drills.length;
-  const preview = data.rest
-    ? "Rest day — nothing to do but rest."
-    : `${load} to work through today${data.topic ? `, starting with ${data.topic.name}` : ""}.`;
+  const load = data.reviews.length + data.drills.length + (data.sweep?.length ?? 0);
+  const lead = data.topics[0]
+    ? `, starting with ${data.topics[0].name}`
+    : data.assigned
+      ? ", starting with today's set"
+      : data.sweep
+        ? " — sweep day, nothing new"
+        : "";
+  const preview = data.rest ? "Rest day — nothing to do but rest." : `${load} to work through today${lead}.`;

  const passed = data.yesterday.total - data.yesterday.failed.length;
  const missed = data.yesterday.failed.map((lc) => `LC ${lc}`).join(", ");
@@ -310,15 +327,47 @@ function DigestEmail({ data }: { data: DigestData }) {

            {data.rest ? null : (
              <>
-                {data.topic ? (
-                  <Section style={{ marginTop: "28px" }}>
-                    <Text style={label}>Something new today</Text>
+                {data.topics.map((topic, i) => (
+                  <Section key={topic.name} style={{ marginTop: i === 0 ? "28px" : "20px" }}>
+                    {i === 0 ? (
+                      <Text style={label}>
+                        {data.topics.length === 1 ? "Something new today" : "Two new things today"}
+                      </Text>
+                    ) : null}
                    <Text style={{ color: INK, fontSize: "16px", fontWeight: 600, margin: "0 0 8px" }}>
-                      {data.topic.name}
+                      {topic.name}
                    </Text>
                    <Text style={hint}>Work through these from the top — a tick means it is already solved.</Text>
-                    <TopicTable rows={data.topic.core} />
+                    <TopicTable rows={topic.core} />
+                    {topic.ifTime.length > 0 ? (
+                      <>
+                        <Text style={{ ...hint, margin: "12px 0 8px" }}>
+                          If time — only once the core is done inside the clock.
+                        </Text>
+                        <TopicTable rows={topic.ifTime} />
+                      </>
+                    ) : null}
                  </Section>
+                ))}
+
+                {data.assigned ? (
+                  <Section style={{ marginTop: "28px" }}>
+                    <Text style={label}>Today's set</Text>
+                    <Text style={hint}>
+                      Picked by number, not by topic — all of them, in order, inside the clock.
+                    </Text>
+                    <TopicTable rows={data.assigned} />
+                  </Section>
+                ) : null}
+
+                {data.sweep ? (
+                  <Block
+                    title="Sweep — redo every red"
+                    note="Every problem still standing on a miss. Nothing new today; clear these first."
+                    rows={data.sweep}
+                    staged
+                    empty="The miss log is clear — nothing is standing on a fail."
+                  />
                ) : null}

                <Block
@@ -409,16 +458,27 @@ function plainDigest(data: DigestData): string {
          : ` · ${data.streak} day${data.streak === 1 ? "" : "s"} in a row, keep it going.`),
  ];

+  const linked = (p: TopicRow) =>
+    `  LC ${p.lc} — ${DIFFICULTY[p.difficulty]?.label ?? p.difficulty}${p.solved ? " (done)" : ""} — ${p.url}`;
+
  if (!data.rest) {
-    if (data.topic) {
+    for (const topic of data.topics) {
      out.push(
        "",
-        `SOMETHING NEW TODAY — ${data.topic.name}`,
+        `SOMETHING NEW TODAY — ${topic.name}`,
        "Work through these from the top; the ones marked done you have already solved.",
-        ...data.topic.core.map(
-          (p) =>
-            `  LC ${p.lc} — ${DIFFICULTY[p.difficulty]?.label ?? p.difficulty}${p.solved ? " (done)" : ""} — ${p.url}`,
-        ),
+        ...topic.core.map(linked),
+      );
+      if (topic.ifTime.length > 0) {
+        out.push("  If time — only once the core is done inside the clock:", ...topic.ifTime.map(linked));
+      }
+    }
+    if (data.assigned) {
+      out.push(
+        "",
+        "TODAY'S SET",
+        "Picked by number, not by topic — all of them, in order, inside the clock.",
+        ...data.assigned.map(linked),
      );
    }

@@ -436,6 +496,14 @@ function plainDigest(data: DigestData): string {
        "No cold starts today.",
      ],
    ];
+    if (data.sweep) {
+      lists.unshift([
+        "SWEEP — REDO EVERY RED",
+        "Every problem still standing on a miss. Nothing new today; clear these first.",
+        data.sweep,
+        "The miss log is clear — nothing is standing on a fail.",
+      ]);
+    }
    for (const [title, note, rows, empty] of lists) {
      out.push("", title);
      if (rows.length === 0) {
@@ -2,21 +2,27 @@
 * Saturday review issue: created at midnight ET so the 8 AM digest can link
 * to it; scored when the issue closes (webhook).
 *
- * The gate half is a topic-blind quiz — one unsolved optional problem per
- * topic scheduled this week (which is why daily drills only draw from
- * earlier weeks). The recap half is generated data, deliberately NOT a task
- * list: the week's solved problems are already scheduled by the +3/+7 ladder,
- * and re-assigning them on Saturday would be massed practice.
+ * The gate half is a topic-blind quiz. When the plan fixes the set
+ * (`gate` in data/schedule.json) that list IS the gate, verbatim — every
+ * number must be a catalog problem, or the issue is not created, because a
+ * `/done` on an uncatalogued number can never be logged. Otherwise the set
+ * is sampled: one unsolved optional problem per topic scheduled this week
+ * (which is why daily drills only draw from earlier weeks), never one the
+ * plan has reserved. The recap half is generated data, deliberately NOT a
+ * task list: the week's solved problems are already scheduled by the +3/+7
+ * ladder, and re-assigning them on Saturday would be massed practice.
 *
 * Label is `review` (the retired Actions system owned `gate`). Creation is
- * idempotent by week; deferred Hards never appear.
+ * idempotent by week; deferred Hards never appear in a sampled set.
 */
 import {
  type ProblemRow,
+  RESERVED,
  type Result,
  SCHEDULE,
  campaignWeek,
  isoWeek,
+  lookupProblems,
  rng,
  sample,
  streak,
@@ -32,7 +38,7 @@ function capitalized(difficulty: string): string {
  return difficulty[0]!.toUpperCase() + difficulty.slice(1);
 }

-/** Unsolved problems of a topic in a set — never attempted, never drilled. */
+/** Unsolved problems of a topic in a set — never attempted, drilled, or reserved. */
 async function freshPool(db: D1Database, topic: number, set: string): Promise<ProblemRow[]> {
  const { results } = await db
    .prepare(
@@ -44,7 +50,7 @@ async function freshPool(db: D1Database, topic: number, set: string): Promise<Pr
    )
    .bind(topic, set)
    .all<ProblemRow>();
-  return results;
+  return results.filter((p) => !RESERVED.has(p.lc_number));
 }

 export interface CreateReport {
@@ -67,40 +73,52 @@ export async function createReviewIssue(env: Env, gh: GitHub, date: string): Pro
    return { created: false, issue: existing.issue, reason: `${title} exists (#${existing.issue})` };
  }

-  // Topics scheduled this week; review-only weeks (Sep 23 onward) sample
-  // across every topic that still has unsolved optional problems.
-  let topics = Object.entries(SCHEDULE)
-    .filter(([d]) => campaignWeek(d) === week)
-    .sort(([a], [b]) => (a < b ? -1 : 1))
-    .map(([, t]) => t);
-  if (topics.length === 0) {
-    const { results } = await db
-      .prepare(
-        `SELECT DISTINCT topic_issue AS t FROM problems
-         WHERE set_label = 'optional' AND stage = 'new'
-           AND lc_number NOT IN (SELECT lc_number FROM drill_pool_used)
-         ORDER BY topic_issue`,
-      )
-      .all<{ t: number }>();
-    topics = sample(results.map((r) => r.t), MAX_GATE_PROBLEMS, rng(`gate-topics-${iso}`)).sort(
-      (a, b) => a - b,
-    );
-  }
-  if (topics.length === 0) return { created: false, reason: "no topics with unsolved pools" };
-
-  const random = rng(`gate-${iso}`);
+  // The plan's fixed set wins; otherwise sample this week's topics, and in a
+  // week without topics, across every topic that still has an unsolved pool.
  const picks: { p: ProblemRow; fallback: boolean }[] = [];
-  for (const topic of topics) {
-    const optional = await freshPool(db, topic, "optional");
-    if (optional.length > 0) {
-      picks.push({ p: sample(optional, 1, random)[0]!, fallback: false });
-      continue;
+  const planned = SCHEDULE[date]?.gate;
+  if (planned) {
+    const { rows, missing } = await lookupProblems(db, planned);
+    if (missing.length) {
+      return {
+        created: false,
+        reason: `planned gate names problems outside the catalog: LC ${missing.join(", LC ")}`,
+      };
    }
-    const core = await freshPool(db, topic, "core");
-    if (core.length > 0) picks.push({ p: sample(core, 1, random)[0]!, fallback: true });
-    // Both pools exhausted: the topic is fully solved; nothing to quiz.
+    picks.push(...rows.map((p) => ({ p, fallback: false })));
+  } else {
+    let topics = Object.entries(SCHEDULE)
+      .filter(([d]) => campaignWeek(d) === week)
+      .sort(([a], [b]) => (a < b ? -1 : 1))
+      .flatMap(([, day]) => day.topics ?? []);
+    if (topics.length === 0) {
+      const { results } = await db
+        .prepare(
+          `SELECT DISTINCT topic_issue AS t FROM problems
+           WHERE set_label = 'optional' AND stage = 'new'
+             AND lc_number NOT IN (SELECT lc_number FROM drill_pool_used)
+           ORDER BY topic_issue`,
+        )
+        .all<{ t: number }>();
+      topics = sample(results.map((r) => r.t), MAX_GATE_PROBLEMS, rng(`gate-topics-${iso}`)).sort(
+        (a, b) => a - b,
+      );
+    }
+    if (topics.length === 0) return { created: false, reason: "no topics with unsolved pools" };
+
+    const random = rng(`gate-${iso}`);
+    for (const topic of topics) {
+      const optional = await freshPool(db, topic, "optional");
+      if (optional.length > 0) {
+        picks.push({ p: sample(optional, 1, random)[0]!, fallback: false });
+        continue;
+      }
+      const core = await freshPool(db, topic, "core");
+      if (core.length > 0) picks.push({ p: sample(core, 1, random)[0]!, fallback: true });
+      // Both pools exhausted: the topic is fully solved; nothing to quiz.
+    }
+    if (picks.length === 0) return { created: false, reason: `every pool for week ${week} is exhausted` };
  }
-  if (picks.length === 0) return { created: false, reason: `every pool for week ${week} is exhausted` };

  // Recap: everything attempted this week, plus streak — data, not tasks.
  const monday = weekMonday(date);
@@ -154,7 +172,7 @@ export async function createReviewIssue(env: Env, gh: GitHub, date: string): Pro

  const milestone = await db
    .prepare("SELECT milestone FROM topics WHERE issue = ? AND milestone IS NOT NULL")
-    .bind(topics[0])
+    .bind(picks[0]!.p.topic_issue)
    .first<{ milestone: number }>();

  const created = (await gh.rest(`/repos/${gh.repo}/issues`, {
@@ -8,6 +8,8 @@ import { describe, expect, test } from "bun:test";

 import {
  INTERVAL,
+  RESERVED,
+  SCHEDULE,
  STAGES,
  WINDOWS,
  addDays,
@@ -19,6 +21,9 @@ import {
  isoWeek,
  rng,
  sample,
+  TEMPERATURES,
+  temperatureOf,
+  topicDate,
  weekdayOf,
  workingDay,
 } from "./srs.ts";
@@ -123,6 +128,38 @@ describe("the rest day", () => {
  });
 });

+describe("the plan", () => {
+  test("never books the rest day, and gates only on Saturday", () => {
+    for (const [date, day] of Object.entries(SCHEDULE)) {
+      expect(isRestDay(date)).toBe(false);
+      if (day.gate) expect(weekdayOf(date)).toBe(6);
+      if (day.topics) expect(weekdayOf(date)).toBeLessThan(6);
+    }
+  });
+
+  test("a topic is taught once; a doubled day answers for both", () => {
+    const seen = new Set<number>();
+    for (const day of Object.values(SCHEDULE)) {
+      for (const topic of day.topics ?? []) {
+        expect(seen.has(topic)).toBe(false);
+        seen.add(topic);
+      }
+    }
+    expect(topicDate(17)).toBe("2026-09-21");
+    expect(topicDate(18)).toBe("2026-09-21");
+    expect(topicDate(999)).toBeUndefined();
+  });
+
+  test("every gate and set problem is reserved from the samplers", () => {
+    for (const day of Object.values(SCHEDULE)) {
+      for (const lc of [...(day.gate ?? []), ...(day.problems ?? [])]) {
+        expect(RESERVED.has(lc)).toBe(true);
+      }
+    }
+    expect(RESERVED.size).toBeGreaterThan(0);
+  });
+});
+
 describe("the ladder", () => {
  test("two rungs, named after the windows", () => {
    expect(STAGES).toEqual(["new", "+3", "+7", "retired"]);
@@ -132,6 +169,31 @@ describe("the ladder", () => {
  });
 });

+describe("concept temperature", () => {
+  test("the bands are the ladder's own windows", () => {
+    // Boundaries are inclusive, and the bands tile the whole age axis with no
+    // gap: whatever the windows become, every age still lands in exactly one.
+    expect(TEMPERATURES).toEqual(["hot", "fresh", "fading", "cold"]);
+    expect(temperatureOf(0)).toBe("hot");
+    expect(temperatureOf(WINDOWS[0])).toBe("hot");
+    expect(temperatureOf(WINDOWS[0] + 1)).toBe("fresh");
+    expect(temperatureOf(WINDOWS[1])).toBe("fresh");
+    expect(temperatureOf(WINDOWS[1] + 1)).toBe("fading");
+    expect(temperatureOf(WINDOWS[1] * 2)).toBe("fading");
+    expect(temperatureOf(WINDOWS[1] * 2 + 1)).toBe("cold");
+  });
+
+  test("never attempted is cold, not unknown", () => {
+    expect(temperatureOf(null)).toBe("cold");
+  });
+
+  test("every age of the campaign has exactly one band", () => {
+    for (let age = 0; age <= CALENDAR.length; age++) {
+      expect(TEMPERATURES).toContain(temperatureOf(age));
+    }
+  });
+});
+
 describe("deterministic sampling", () => {
  test("same seed, same picks", () => {
    const pool = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
@@ -1,6 +1,6 @@
 /**
- * SRS domain: ET dates, the work week, the interval ladder, deterministic
- * sampling, and the one write path for attempts.
+ * SRS domain: ET dates, the work week, the interval ladder, concept
+ * temperature, deterministic sampling, and the one write path for attempts.
 *
 * D1 is the single source of truth. The stage names the review a problem must
 * pass NEXT (`+3` = due 3 working days after the last clean solve). Passing
@@ -126,16 +126,50 @@ export function prettyDate(date: string): string {

 // ── the schedule (bundled; humans edit apps/api/data/schedule.json) ───

-export const SCHEDULE: Record<string, number> = scheduleJson;
+/**
+ * One calendar day of the plan. Most days teach topics (one, or two when the
+ * plan doubles up); the rest are named by what they assign instead: a fixed
+ * gate set for Saturday's review issue, a fixed problem set (a "B-set" day),
+ * or a sweep — redo every miss, nothing new. Absent dates are unplanned:
+ * the ladder still runs, the digest just has nothing new to say.
+ */
+export interface ScheduledDay {
+  /** Topic issues taught that day, in the order the digest presents them. */
+  topics?: number[];
+  /** Problems assigned by number instead of by topic. */
+  problems?: number[];
+  /** Saturday: the blind set, fixed by the plan rather than sampled. */
+  gate?: number[];
+  /** Miss-log sweep: every problem whose last attempt failed, and no new ones. */
+  sweep?: boolean;
+}

-/** Week in which a topic was (or will be) taught. */
-export function topicWeek(topic: number): number | undefined {
-  for (const [date, t] of Object.entries(SCHEDULE)) {
-    if (t === topic) return campaignWeek(date);
+export const SCHEDULE: Record<string, ScheduledDay> = scheduleJson;
+
+/** Date the schedule teaches a topic on, if it teaches it at all. */
+export function topicDate(topic: number): string | undefined {
+  for (const [date, day] of Object.entries(SCHEDULE)) {
+    if (day.topics?.includes(topic)) return date;
  }
  return undefined;
 }

+/** Week in which a topic was (or will be) taught. */
+export function topicWeek(topic: number): number | undefined {
+  const date = topicDate(topic);
+  return date === undefined ? undefined : campaignWeek(date);
+}
+
+/**
+ * Every problem the plan names outright — a gate's blind set or a set day's
+ * list. The plan owns these: the samplers (daily drills, a sampled gate)
+ * never draw them, so a gate problem stays unseen until its Saturday and a
+ * set day's problems wait for their day.
+ */
+export const RESERVED: ReadonlySet<number> = new Set(
+  Object.values(SCHEDULE).flatMap((day) => [...(day.gate ?? []), ...(day.problems ?? [])]),
+);
+
 // ── deterministic sampling ───────────────────────────────────────

 /** FNV-1a → mulberry32: seeded PRNG so re-runs pick identical problems. */
@@ -182,6 +216,45 @@ export const STAGES: Stage[] = ["new", "+3", "+7", "retired"];
 export type Result = "pass" | "fail";
 export type Kind = "first" | "review" | "drill" | "gate";

+/**
+ * Catalog vocabulary, carried by the `diff:*` / `set:*` issue labels and
+ * stored verbatim in `problems`. Listed low to high / most to least required,
+ * which is the order every chart, table and digest section prints them in.
+ */
+export const DIFFICULTIES = ["easy", "medium", "hard"] as const;
+export type Difficulty = (typeof DIFFICULTIES)[number];
+export const SETS = ["core", "optional", "deferred"] as const;
+export type SetLabel = (typeof SETS)[number];
+
+/**
+ * How warm a concept is: the age of a topic's most recent attempt, bucketed by
+ * the ladder's own windows so the two cannot drift. Touched inside the first
+ * window it is `hot`, inside the second `fresh`, inside one more full second
+ * window `fading`; older than that — or never attempted — it is `cold`.
+ *
+ * A read-side label only: nothing is scheduled off it. It answers the question
+ * the ladder cannot, because the ladder tracks problems and a topic can go
+ * quiet for a fortnight while not one of its problems comes due.
+ */
+export const TEMPERATURES = ["hot", "fresh", "fading", "cold"] as const;
+export type Temperature = (typeof TEMPERATURES)[number];
+
+/** Warmest band first, with the age it tolerates. Past the last one is cold. */
+const TEMPERATURE_MAX_AGE: [Temperature, number][] = [
+  ["hot", WINDOWS[0]],
+  ["fresh", WINDOWS[1]],
+  ["fading", WINDOWS[1] * 2],
+];
+
+/** Days since a topic's last attempt → its band; null (never) is cold. */
+export function temperatureOf(age: number | null): Temperature {
+  if (age === null) return "cold";
+  for (const [temperature, max] of TEMPERATURE_MAX_AGE) {
+    if (age <= max) return temperature;
+  }
+  return "cold";
+}
+
 export interface ProblemRow {
  lc_number: number;
  issue: number;
@@ -203,6 +276,31 @@ export async function getProblem(db: D1Database, lc: number): Promise<ProblemRow
  return db.prepare("SELECT * FROM problems WHERE lc_number = ?").bind(lc).first<ProblemRow>();
 }

+/**
+ * Catalog rows for a list of numbers the plan names, in the plan's order,
+ * plus the numbers the catalog does not know — a planned problem without an
+ * issue can never be logged, so callers decide whether that is fatal.
+ */
+export async function lookupProblems(
+  db: D1Database,
+  lcs: number[],
+): Promise<{ rows: ProblemRow[]; missing: number[] }> {
+  if (lcs.length === 0) return { rows: [], missing: [] };
+  const { results } = await db
+    .prepare(`SELECT * FROM problems WHERE lc_number IN (${lcs.map(() => "?").join(",")})`)
+    .bind(...lcs)
+    .all<ProblemRow>();
+  const byNumber = new Map(results.map((p) => [p.lc_number, p]));
+  const rows: ProblemRow[] = [];
+  const missing: number[] = [];
+  for (const lc of lcs) {
+    const row = byNumber.get(lc);
+    if (row) rows.push(row);
+    else missing.push(lc);
+  }
+  return { rows, missing };
+}
+
 /**
 * What "due" means, in one place: not retired, scheduled on/before ?1, and
 * past its deferral if it has one. Every caller that asks "what is open on
@@ -313,8 +411,9 @@ export async function levelReviews(db: D1Database, date: string): Promise<number
 * scheduled in an earlier week (the current week's optional pool is reserved
 * for Saturday's gate) AND showing learning evidence: at least one of the
 * topic's core problems has entered the ladder. A skipped learning day never
- * feeds drills just because its calendar week lapsed. Never repeated,
- * seeded by date. Boosted topics contribute up to 2 extra.
+ * feeds drills just because its calendar week lapsed. Problems the plan names
+ * outright (RESERVED) are never drawn. Never repeated, seeded by date.
+ * Boosted topics contribute up to 2 extra.
 */
 export async function pickDrills(
  db: D1Database,
@@ -337,6 +436,7 @@ export async function pickDrills(
    )
    .all<ProblemRow & { boost: number }>();
  const pool = results.filter((p) => {
+    if (RESERVED.has(p.lc_number)) return false;
    const w = topicWeek(p.topic_issue);
    return w !== undefined && w < week;
  });
@@ -1,19 +1,29 @@
 /**
- * GET /api/stats — the one JSON document behind the docs progress page.
- * Read-only aggregation over D1; shape is the page's contract, change both
- * together. A problem counts as "done" once it entered the ladder
- * (stage != 'new').
+ * GET /api/stats — the one JSON document behind the docs progress page and the
+ * `bun run stats` terminal dashboard. Read-only aggregation over D1; the shape
+ * is both readers' contract, so change all three together. A problem counts as
+ * "done" once it entered the ladder (stage != 'new').
+ *
+ * The temperature block is the one figure here that is not a count of rows:
+ * srs.ts turns a topic's days-since-last-attempt into a band, so "which
+ * concepts have gone cold" has exactly one definition and the dashboard is
+ * free to colour it however it likes.
 */
 import {
  CAMPAIGN_DAYS,
  CAMPAIGN_START,
+  DIFFICULTIES,
  DUE_WHERE,
  addDays,
  campaignDay,
  campaignWeek,
+  daysBetween,
  isoWeek,
  STAGES,
  streak,
+  temperatureOf,
+  topicDate,
+  type Temperature,
 } from "./srs.ts";

 const PHASE_NAMES: Record<number, string> = {
@@ -24,6 +34,91 @@ const PHASE_NAMES: Record<number, string> = {
  5: "V — Decision Space",
 };

+/** One concept: catalog coverage plus how long since it was last exercised. */
+interface TopicStat {
+  issue: number;
+  name: string;
+  milestone: number | null;
+  /** Campaign week the schedule teaches it in; null when unscheduled. */
+  week: number | null;
+  /** False while the schedule has not reached it — temperature is moot. */
+  taught: boolean;
+  total: number;
+  done: number;
+  retired: number;
+  due: number;
+  /** ET date of the most recent attempt on any of its problems. */
+  last: string | null;
+  /** Days since `last`; null when never attempted. */
+  age: number | null;
+  temperature: Temperature;
+}
+
+async function buildTopics(db: D1Database, today: string): Promise<TopicStat[]> {
+  // Three shapes, three statements: batch() carries one row type, and casting
+  // its results apart would cost more than the round trips it saves.
+  const { results: coverage } = await db
+    .prepare(
+      `SELECT t.issue AS issue, t.name AS name, t.milestone AS milestone,
+              COUNT(p.lc_number) AS total,
+              SUM(CASE WHEN p.stage != 'new' THEN 1 ELSE 0 END) AS done,
+              SUM(CASE WHEN p.stage = 'retired' THEN 1 ELSE 0 END) AS retired
+       FROM topics t LEFT JOIN problems p ON p.topic_issue = t.issue
+       GROUP BY t.issue
+       ORDER BY t.issue`,
+    )
+    .all<{
+      issue: number;
+      name: string;
+      milestone: number | null;
+      total: number;
+      done: number | null;
+      retired: number | null;
+    }>();
+
+  const { results: activity } = await db
+    .prepare(
+      `SELECT p.topic_issue AS issue, MAX(a.date) AS last
+       FROM attempts a JOIN problems p ON p.lc_number = a.lc_number
+       GROUP BY p.topic_issue`,
+    )
+    .all<{ issue: number; last: string | null }>();
+  const lastByTopic = new Map<number, string>();
+  for (const row of activity) {
+    if (row.last) lastByTopic.set(row.issue, row.last);
+  }
+
+  const { results: dueRows } = await db
+    .prepare(
+      `SELECT topic_issue AS issue, COUNT(*) AS n
+       FROM problems WHERE ${DUE_WHERE} GROUP BY topic_issue`,
+    )
+    .bind(today)
+    .all<{ issue: number; n: number }>();
+  const dueByTopic = new Map<number, number>();
+  for (const row of dueRows) dueByTopic.set(row.issue, row.n);
+
+  return coverage.map((row) => {
+    const taughtOn = topicDate(row.issue);
+    const last = lastByTopic.get(row.issue) ?? null;
+    const age = last === null ? null : daysBetween(last, today);
+    return {
+      issue: row.issue,
+      name: row.name,
+      milestone: row.milestone,
+      week: taughtOn === undefined ? null : campaignWeek(taughtOn),
+      taught: taughtOn !== undefined && taughtOn <= today,
+      total: row.total,
+      done: row.done ?? 0,
+      retired: row.retired ?? 0,
+      due: dueByTopic.get(row.issue) ?? 0,
+      last,
+      age,
+      temperature: temperatureOf(age),
+    };
+  });
+}
+
 export async function buildStats(db: D1Database, today: string): Promise<object> {
  const { results: phaseRows } = await db
    .prepare(
@@ -65,6 +160,21 @@ export async function buildStats(db: D1Database, today: string): Promise<object>
  const ladder: Record<string, number> = Object.fromEntries(STAGES.map((s) => [s, 0]));
  for (const row of ladderRows) ladder[row.stage] = row.n;

+  // Same shape as `ladder`: every band present, zeros included, in DIFFICULTIES
+  // order so a reader can index it without sorting.
+  const { results: diffRows } = await db
+    .prepare(
+      `SELECT difficulty,
+              COUNT(*) AS total,
+              SUM(CASE WHEN stage != 'new' THEN 1 ELSE 0 END) AS done
+       FROM problems GROUP BY difficulty`,
+    )
+    .all<{ difficulty: string; total: number; done: number }>();
+  const difficulty = DIFFICULTIES.map((name) => {
+    const row = diffRows.find((r) => r.difficulty === name);
+    return { difficulty: name, total: row?.total ?? 0, done: row?.done ?? 0 };
+  });
+
  // gates rows are keyed by ISO week (that's the digest/gate contract), but
  // the page speaks campaign weeks. The campaign never crosses a year
  // boundary, so a plain offset converts safely.
@@ -74,39 +184,47 @@ export async function buildStats(db: D1Database, today: string): Promise<object>
    .all<{ week: number; issue: number | null; pass_rate: number | null; closed_on: string | null }>();
  const gates = gateRows.map((g) => ({ ...g, week: g.week - isoWeekOffset }));

-  // Review-queue depth for the next 14 days: everything due by that day.
-  const queue: { date: string; due: number }[] = [];
-  for (let i = 0; i < 14; i++) {
-    const date = addDays(today, i);
-    const row = await db
-      .prepare(`SELECT COUNT(*) AS n FROM problems WHERE ${DUE_WHERE}`)
-      .bind(date)
-      .first<{ n: number }>();
-    queue.push({ date, due: row?.n ?? 0 });
-  }
+  // Review-queue depth for the next 14 days: everything due by that day. One
+  // statement per day, but DUE_WHERE stays the only definition of "due" — the
+  // batch is a single round trip, not fourteen.
+  const dates = Array.from({ length: 14 }, (_, i) => addDays(today, i));
+  const queueRows = await db.batch<{ n: number }>(
+    dates.map((date) =>
+      db.prepare(`SELECT COUNT(*) AS n FROM problems WHERE ${DUE_WHERE}`).bind(date),
+    ),
+  );
+  const queue = dates.map((date, i) => ({ date, due: queueRows[i]?.results[0]?.n ?? 0 }));

-  const recent: { date: string; attempts: number; passes: number }[] = [];
-  for (let i = 6; i >= 0; i--) {
-    const date = addDays(today, -i);
-    const row = await db
-      .prepare(
-        `SELECT COUNT(*) AS attempts,
-                SUM(CASE WHEN result = 'pass' THEN 1 ELSE 0 END) AS passes
-         FROM attempts WHERE date = ?`,
-      )
-      .bind(date)
-      .first<{ attempts: number; passes: number | null }>();
-    recent.push({ date, attempts: row?.attempts ?? 0, passes: row?.passes ?? 0 });
-  }
+  // Attempts per campaign day, every day present. This is the heatmap's data
+  // model — the docs page reads the tail of it for "last 7 days" rather than
+  // asking a second question that could answer differently.
+  const end = addDays(CAMPAIGN_START, CAMPAIGN_DAYS - 1);
+  const { results: heatRows } = await db
+    .prepare(
+      `SELECT date,
+              COUNT(*) AS attempts,
+              SUM(CASE WHEN result = 'pass' THEN 1 ELSE 0 END) AS passes
+       FROM attempts WHERE date BETWEEN ?1 AND ?2 GROUP BY date`,
+    )
+    .bind(CAMPAIGN_START, end)
+    .all<{ date: string; attempts: number; passes: number | null }>();
+  const byDate = new Map(heatRows.map((r) => [r.date, r]));
+  const heat = Array.from({ length: CAMPAIGN_DAYS }, (_, i) => {
+    const date = addDays(CAMPAIGN_START, i);
+    const row = byDate.get(date);
+    return { date, attempts: row?.attempts ?? 0, passes: row?.passes ?? 0 };
+  });

  return {
    generated: new Date().toISOString(),
    campaign: { day: campaignDay(today), week: campaignWeek(today), start: CAMPAIGN_START, days: CAMPAIGN_DAYS },
    phases: [...phases.values()],
    ladder,
+    difficulty,
+    topics: await buildTopics(db, today),
    gates,
    streak: await streak(db, today),
    queue,
-    recent,
+    heat,
  };
 }
@@ -0,0 +1,87 @@
+/**
+ * The SRS Worker, from the CLI side: one base URL and one read-side payload.
+ *
+ * `SRS_API` points a script at a `wrangler dev` instance instead of production
+ * — the same switch close-solved.ts uses for its `/admin/solved` push, which is
+ * why the default lives here rather than in either script.
+ *
+ * The payload is narrowed with a guard rather than cast: D1 is the only source
+ * of SRS truth, so if the Worker answers something else the caller must say so
+ * out loud, not render half a dashboard off undefined fields.
+ */
+import { STAGES, TEMPERATURES, type Stage, type Temperature } from "../api/src/srs.ts";
+
+export const SRS_API = process.env.SRS_API ?? "https://srs-api.prdlk.workers.dev";
+
+// The rungs and the bands come straight from the Worker's domain module — the
+// same import work.ts makes for WINDOWS — so a change to either vocabulary
+// reaches this dashboard without a second list to remember.
+export { STAGES, TEMPERATURES, type Stage, type Temperature };
+
+export interface Phase {
+  milestone: number;
+  name: string;
+  core_done: number;
+  core_total: number;
+  optional_done: number;
+  optional_total: number;
+  deferred_done: number;
+  deferred_total: number;
+}
+
+export interface Topic {
+  issue: number;
+  name: string;
+  milestone: number | null;
+  week: number | null;
+  taught: boolean;
+  total: number;
+  done: number;
+  retired: number;
+  due: number;
+  last: string | null;
+  age: number | null;
+  temperature: Temperature;
+}
+
+export interface Stats {
+  generated: string;
+  campaign: { day: number; week: number; start: string; days: number };
+  phases: Phase[];
+  ladder: Record<Stage, number>;
+  difficulty: { difficulty: string; total: number; done: number }[];
+  topics: Topic[];
+  /** week is the 1-based campaign week — the Worker converts from ISO. */
+  gates: { week: number; issue: number | null; pass_rate: number | null; closed_on: string | null }[];
+  streak: number;
+  queue: { date: string; due: number }[];
+  /** Every campaign day, zeros and future days included. */
+  heat: { date: string; attempts: number; passes: number }[];
+}
+
+function isStats(value: unknown): value is Stats {
+  if (typeof value !== "object" || value === null) return false;
+  const v = value as Record<string, unknown>;
+  return (
+    typeof v.campaign === "object" &&
+    v.campaign !== null &&
+    typeof v.ladder === "object" &&
+    v.ladder !== null &&
+    typeof v.streak === "number" &&
+    Array.isArray(v.phases) &&
+    Array.isArray(v.difficulty) &&
+    Array.isArray(v.topics) &&
+    Array.isArray(v.gates) &&
+    Array.isArray(v.queue) &&
+    Array.isArray(v.heat)
+  );
+}
+
+/** GET /api/stats. Throws with the reason; there is no local substitute. */
+export async function fetchStats(signal?: AbortSignal): Promise<Stats> {
+  const res = await fetch(`${SRS_API}/api/stats`, { signal });
+  if (!res.ok) throw new Error(`GET /api/stats -> ${res.status} ${await res.text()}`);
+  const body: unknown = await res.json();
+  if (!isStats(body)) throw new Error("GET /api/stats -> unrecognised payload shape");
+  return body;
+}
@@ -21,6 +21,7 @@
 */
 import { join } from "node:path";

+import { SRS_API } from "./api.ts";
 import { github } from "./github.ts";
 import { markdownTable, printTable, writeStepSummary } from "./report.ts";
 import { isImplemented } from "./source.ts";
@@ -162,7 +163,8 @@ for (const num of [...work.keys()].sort((a, b) => a - b)) {
 // rung named, so the whole implemented set can go over on every push (a total
 // recompute that backfills whatever earlier runs missed) and re-runs write
 // nothing.
-const SRS_API = process.env.SRS_API ?? "https://srs-api.prdlk.workers.dev";
+// SRS_API (apps/cli/api.ts) is the switch that points this at a wrangler dev
+// instance instead of production.
 const solved = [...work.entries()]
  .flatMap(([lc, entry]) => [...entry.solved].sort().map((bucket) => ({ lc, bucket })))
  .sort((a, b) => a.lc - b.lc || a.bucket - b.bucket);
@@ -4,15 +4,17 @@
  "type": "module",
  "scripts": {
    "pick": "bun ./pick.ts",
+    "stats": "bun ./stats.ts",
    "test": "bun ./test.ts",
    "submit": "bun ./submit.ts",
    "sync": "bun ./sync.ts",
+    "videos": "bun ./videos.ts",
    "close-solved": "bun ./close-solved.ts",
    "close-topics": "bun ./close-topics.ts"
  },
  "devDependencies": {
    "@clack/prompts": "^1.7.0",
-    "@night-slayer18/leetcode-cli": "^3.5.0",
+    "@night-slayer18/leetcode-cli": "^3.5.2",
    "@types/bun": "^1.3.14"
  }
 }
@@ -30,17 +30,27 @@ function lcNumber(rel: string): number | undefined {
 }

 /** Fuzzy-pick a solution under work/ and run `leetcode <command>` on it. */
-export async function runOnSolution(command: "test" | "submit", message: string): Promise<never> {
+export async function runOnSolution(
+  command: "test" | "submit",
+  message: string,
+): Promise<never> {
  const db = problemDb();

-  const scanned = [...new Bun.Glob("**/*.{js,ts,py,java,c,cpp,go,rs,rb,swift,kt,cs}").scanSync({ cwd: WORK_DIR })];
+  const scanned = [
+    ...new Bun.Glob("**/*.{js,ts,py,java,c,cpp,go,rs,rb,swift,kt,cs}").scanSync(
+      { cwd: WORK_DIR },
+    ),
+  ];

  // register creation times so files older than the db still sort correctly
  for (const f of scanned) {
    const num = lcNumber(f);
    if (num === undefined) continue;
    const stat = statSync(join(WORK_DIR, f));
-    db.ensureSolution(num, new Date(stat.birthtimeMs || stat.mtimeMs).toISOString());
+    db.ensureSolution(
+      num,
+      new Date(stat.birthtimeMs || stat.mtimeMs).toISOString(),
+    );
  }
  const activity = db.solutionActivity();

@@ -51,20 +61,21 @@ export async function runOnSolution(command: "test" | "submit", message: string)
    return (command === "test" ? row?.lastTested : row?.createdAt) ?? undefined;
  };

-  const files = scanned
-    .sort((a, b) => {
-      const ka = sortKey(a);
-      const kb = sortKey(b);
-      if (ka !== kb) {
-        if (ka === undefined) return 1;
-        if (kb === undefined) return -1;
-        return kb.localeCompare(ka); // ISO timestamps: lexicographic = chronological
-      }
-      return a.localeCompare(b, undefined, { numeric: true });
-    });
+  const files = scanned.sort((a, b) => {
+    const ka = sortKey(a);
+    const kb = sortKey(b);
+    if (ka !== kb) {
+      if (ka === undefined) return 1;
+      if (kb === undefined) return -1;
+      return kb.localeCompare(ka); // ISO timestamps: lexicographic = chronological
+    }
+    return a.localeCompare(b, undefined, { numeric: true });
+  });

  if (files.length === 0) {
-    log.error(`No solution files found in ${relative(process.cwd(), WORK_DIR)}/`);
+    log.error(
+      `No solution files found in ${relative(process.cwd(), WORK_DIR)}/`,
+    );
    process.exit(1);
  }

@@ -74,11 +85,14 @@ export async function runOnSolution(command: "test" | "submit", message: string)
    maxItems: 12,
    options: files.map((f) => {
      const parsed = parseWorkPath(f);
-      const stage = parsed?.bucket === 1 ? "first solve" : `${parsed?.bucket}-day review`;
+      const stage =
+        parsed?.bucket === 1 ? "first solve" : `${parsed?.bucket}-day review`;
      return {
        value: f,
        label: basename(f),
-        hint: parsed ? `${stage} · ${parsed.difficulty} · ${parsed.category}` : f,
+        hint: parsed
+          ? `${stage} · ${parsed.difficulty} · ${parsed.category}`
+          : f,
      };
    }),
    filter: (search, option) => {
@@ -0,0 +1,458 @@
+#!/usr/bin/env bun
+/**
+ * `bun run stats` — the campaign dashboard, in the terminal.
+ *
+ * Four panes over ONE source: GET /api/stats. D1 owns SRS state, so this
+ * fetches the Worker's aggregate rather than recomputing anything from work/
+ * or from git — a second answer to "how far along am I" is exactly the drift
+ * the campaign is built to avoid. The one local fact shown is the work/ file
+ * inventory, which is inventory, not schedule. Unreachable Worker = an error,
+ * never a reconstructed guess.
+ *
+ * The panes:
+ *   Overview  campaign clock, solved counts, phase bars, recent activity
+ *   Ladder    the +3/+7 rungs and the 14-day due queue
+ *   Concepts  every topic by temperature — what has gone cold
+ *   Activity  the campaign heatmap and the weekly gates
+ *
+ * Outside a TTY (a pipe, CI) it prints every pane once and exits, so the same
+ * command works in a scrollback or a file.
+ */
+import {
+  fetchStats,
+  SRS_API,
+  STAGES,
+  TEMPERATURES,
+  type Stats,
+  type Temperature,
+  type Topic,
+} from "./api.ts";
+import {
+  BLUE,
+  FG,
+  GREEN,
+  GREENS,
+  LINE,
+  MUTED,
+  RED,
+  YELLOW,
+  bar,
+  bold,
+  cell,
+  fg,
+  heatCell,
+  openScreen,
+  padTo,
+  rule,
+  sparkline,
+  stackedBar,
+  width,
+} from "./tui.ts";
+import { BUCKETS, WINDOWS, solvedInBucket } from "./work.ts";
+
+// ── shared vocabulary → colour ───────────────────────────────────
+
+// Temperature is an action ramp, not a thermometer: green is warm enough to
+// leave alone, red is the concept that wants revisiting. The band names carry
+// the heat, the colours carry the urgency — and the glyphs carry both again,
+// so a piped or NO_COLOR dashboard still separates the bands.
+const TEMP_COLOR: Record<Temperature, string> = {
+  hot: GREENS[4]!,
+  fresh: GREEN,
+  fading: YELLOW,
+  cold: RED,
+};
+
+const TEMP_GLYPH: Record<Temperature, string> = {
+  hot: "█",
+  fresh: "▓",
+  fading: "▒",
+  cold: "░",
+};
+
+const DIFF_COLOR: Record<string, string> = { easy: GREEN, medium: YELLOW, hard: RED };
+
+const LABEL = 15; // left-hand label column, shared by every pane
+
+/** "2026-08-31" → "Mon 31", read as a plain calendar date, never localised. */
+function dayLabel(iso: string): string {
+  const [y, m, d] = iso.split("-").map(Number);
+  const weekday = new Date(Date.UTC(y!, m! - 1, d!)).getUTCDay();
+  return `${["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"][weekday]} ${String(d).padStart(2, "0")}`;
+}
+
+// ── overview ─────────────────────────────────────────────────────
+
+function overview(s: Stats, cols: number): string[] {
+  const bars = Math.max(10, Math.min(48, cols - LABEL - 18));
+  const lines: string[] = [];
+
+  const { day, days, week } = s.campaign;
+  lines.push(rule("campaign", cols));
+  lines.push(
+    `${cell("Day", LABEL)}${bar(day, days, bars)}  ${fg(FG, `${day}/${days}`)}` +
+      fg(MUTED, `  week ${week}`),
+  );
+  const streak = s.streak > 0 ? fg(GREENS[4]!, `${s.streak} day${s.streak === 1 ? "" : "s"}`) : fg(MUTED, "none");
+  lines.push(`${cell("Streak", LABEL)}${streak}`);
+
+  const total = s.difficulty.reduce((n, d) => n + d.total, 0);
+  const done = s.difficulty.reduce((n, d) => n + d.done, 0);
+  lines.push("");
+  lines.push(rule("solved", cols));
+  lines.push(
+    `${cell("All", LABEL)}${bar(done, total, bars)}  ${fg(FG, `${done}/${total}`)}` +
+      fg(MUTED, `  ${total > 0 ? Math.round((done / total) * 100) : 0}%`),
+  );
+  for (const d of s.difficulty) {
+    lines.push(
+      `${cell(d.difficulty, LABEL)}${bar(d.done, d.total, bars, DIFF_COLOR[d.difficulty] ?? GREEN)}` +
+        `  ${fg(MUTED, `${d.done}/${d.total}`)}`,
+    );
+  }
+
+  lines.push("");
+  lines.push(rule("phases", cols));
+  lines.push(
+    `${" ".repeat(LABEL)}${fg(GREEN, "█")} ${fg(MUTED, "core")}  ${fg(BLUE, "█")} ${fg(MUTED, "optional")}  ` +
+      `${fg(RED, "█")} ${fg(MUTED, "deferred")}  ${fg(LINE, "░")} ${fg(MUTED, "remaining")}`,
+  );
+  // Every phase bar is scaled to the LARGEST phase, not to itself: phases are
+  // not the same size, and equal-length bars would hide that.
+  const scale = Math.max(
+    1,
+    ...s.phases.map((p) => p.core_total + p.optional_total + p.deferred_total),
+  );
+  for (const p of s.phases) {
+    const phaseDone = p.core_done + p.optional_done + p.deferred_done;
+    const phaseTotal = p.core_total + p.optional_total + p.deferred_total;
+    const segments = [
+      { value: p.core_done, color: GREEN },
+      { value: p.optional_done, color: BLUE },
+      { value: p.deferred_done, color: RED },
+      { value: phaseTotal - phaseDone, color: LINE, glyph: "░" },
+    ];
+    lines.push(
+      `${cell(p.name, LABEL - 1)} ${stackedBar(segments, scale, bars)}  ${fg(MUTED, `${phaseDone}/${phaseTotal}`)}`,
+    );
+  }
+
+  lines.push("");
+  lines.push(rule("last 14 days", cols));
+  const window = s.heat.slice(Math.max(0, day - 14), day);
+  const attempts = window.reduce((n, h) => n + h.attempts, 0);
+  const passes = window.reduce((n, h) => n + h.passes, 0);
+  lines.push(
+    `${cell("Attempts", LABEL)}${sparkline(window.map((h) => h.attempts))}  ` +
+      fg(MUTED, `${attempts} attempts · ${passes} passed`),
+  );
+
+  // The one local fact on the dashboard: solutions on disk, per
+  // spaced-repetition bucket. Inventory, not schedule — D1's ladder is the
+  // schedule, and the Ladder pane is where it is shown. Empty scaffolds do not
+  // count, the same rule close-solved.ts closes issues by.
+  lines.push("");
+  lines.push(rule("work/ on disk", cols));
+  lines.push(
+    cell("Solutions", LABEL) +
+      BUCKETS.map((b) => `${fg(FG, String(solvedInBucket(b)))} ${fg(MUTED, `in work/${b}`)}`).join(
+        fg(MUTED, " · "),
+      ),
+  );
+
+  return lines;
+}
+
+// ── ladder ───────────────────────────────────────────────────────
+
+function ladder(s: Stats, cols: number): string[] {
+  const bars = Math.max(10, Math.min(48, cols - LABEL - 18));
+  const lines: string[] = [rule("ladder", cols)];
+  lines.push(fg(MUTED, `${" ".repeat(LABEL)}the review each problem owes next`));
+
+  const rungs = STAGES.map((stage) => s.ladder[stage] ?? 0);
+  const peak = Math.max(1, ...rungs);
+  STAGES.forEach((stage, i) => {
+    const n = rungs[i]!;
+    // new is the untouched pile and retired is the finished one; only the
+    // middle rungs are live work, so only they get the live colour.
+    const color = stage === "new" ? LINE : stage === "retired" ? GREENS[4]! : GREEN;
+    lines.push(`${cell(stage, LABEL)}${bar(n, peak, bars, color)}  ${fg(FG, String(n))}`);
+  });
+
+  lines.push("");
+  lines.push(rule("due queue", cols));
+  const today = s.queue[0];
+  lines.push(
+    fg(MUTED, `${" ".repeat(LABEL)}cumulative: everything the ladder wants by that day`) +
+      (today ? fg(FG, `  ·  ${today.due} due today`) : ""),
+  );
+  const depth = Math.max(1, ...s.queue.map((q) => q.due));
+  for (const q of s.queue) {
+    const color = q.due > 3 ? RED : q.due > 0 ? GREEN : LINE;
+    lines.push(
+      `${cell(dayLabel(q.date), LABEL)}${bar(q.due, depth, bars, color)}  ${fg(MUTED, String(q.due))}`,
+    );
+  }
+
+  return lines;
+}
+
+// ── concepts ─────────────────────────────────────────────────────
+
+/** Coldest first, and everything the schedule has not reached yet last. */
+function byUrgency(a: Topic, b: Topic): number {
+  if (a.taught !== b.taught) return a.taught ? -1 : 1;
+  return (
+    TEMPERATURES.indexOf(b.temperature) - TEMPERATURES.indexOf(a.temperature) ||
+    (b.age ?? Infinity) - (a.age ?? Infinity) ||
+    a.issue - b.issue
+  );
+}
+
+// Row layout: "● " + name + temp + coverage bar + done/total + last + due.
+// Everything but the name is fixed, so the name absorbs the terminal width.
+const TEMP_W = 7;
+const COVER_W = 10;
+const COUNT_W = 6;
+const AGE_W = 6;
+const DUE_W = 3;
+const CONCEPT_FIXED = 3 + (TEMP_W + 1) + (COVER_W + 1) + (COUNT_W + 1) + (AGE_W + 1) + DUE_W + 1;
+
+function concepts(s: Stats, cols: number): string[] {
+  const lines: string[] = [rule("concepts by temperature", cols)];
+  // Thresholds are the ladder's windows; print them rather than restate them.
+  const bands: [Temperature, string][] = [
+    ["hot", `≤${WINDOWS[0]}d`],
+    ["fresh", `≤${WINDOWS[1]}d`],
+    ["fading", `≤${WINDOWS[1] * 2}d`],
+    ["cold", "older, or never"],
+  ];
+  lines.push(
+    fg(MUTED, "  last exercised — ") +
+      bands
+        .map(([t, when]) => `${fg(TEMP_COLOR[t], `${TEMP_GLYPH[t]} ${t}`)} ${fg(MUTED, when)}`)
+        .join(fg(MUTED, " · ")),
+  );
+
+  const taught = s.topics.filter((t) => t.taught);
+  const counts = TEMPERATURES.map((t) => ({
+    value: taught.filter((x) => x.temperature === t).length,
+    color: TEMP_COLOR[t],
+    glyph: TEMP_GLYPH[t],
+  }));
+  lines.push("");
+  lines.push(`  ${stackedBar(counts, Math.max(1, taught.length), Math.max(10, Math.min(60, cols - 4)))}`);
+  lines.push(
+    `  ${TEMPERATURES.map((t, i) => `${fg(TEMP_COLOR[t], TEMP_GLYPH[t])} ${counts[i]!.value} ${fg(MUTED, t)}`).join("  ")}` +
+      fg(MUTED, `   of ${taught.length} taught so far`),
+  );
+
+  const nameW = Math.max(12, Math.min(34, cols - CONCEPT_FIXED));
+  lines.push("");
+  lines.push(
+    fg(
+      MUTED,
+      `   ${cell("concept", nameW)} ${cell("temp", TEMP_W)} ` +
+        `${cell("coverage", COVER_W + 1 + COUNT_W)} ${cell("last", AGE_W)} ${cell("due", DUE_W, "right")}`,
+    ),
+  );
+
+  for (const t of s.topics.slice().sort(byUrgency)) {
+    // An unreached concept has no temperature to report — it shows the week
+    // the schedule gets to it instead, so the list stays one list.
+    const color = t.taught ? TEMP_COLOR[t.temperature] : LINE;
+    const age =
+      !t.taught ? "—" : t.age === null ? "never" : t.age <= 0 ? "today" : `${t.age}d`;
+    lines.push(
+      ` ${fg(color, t.taught ? TEMP_GLYPH[t.temperature] : "·")} ${fg(t.taught ? FG : MUTED, cell(t.name, nameW))} ` +
+        `${fg(color, cell(t.taught ? t.temperature : `wk ${t.week ?? "?"}`, TEMP_W))} ` +
+        `${bar(t.done, t.total, COVER_W, t.taught ? GREEN : LINE)} ` +
+        `${fg(MUTED, cell(`${t.done}/${t.total}`, COUNT_W))} ${fg(MUTED, cell(age, AGE_W))} ` +
+        `${t.due > 0 ? fg(GREEN, cell(String(t.due), DUE_W, "right")) : fg(LINE, cell("·", DUE_W, "right"))}`,
+    );
+  }
+
+  return lines;
+}
+
+// ── activity ─────────────────────────────────────────────────────
+
+const WEEKDAYS = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];
+
+function activity(s: Stats, cols: number): string[] {
+  const lines: string[] = [rule("attempts per campaign day", cols)];
+  lines.push(fg(MUTED, "  the README heatmap, same ramp — Sunday is the rest day"));
+  lines.push("");
+
+  // CAMPAIGN_START is a Monday, so the grid is simply the series in weeks of
+  // seven: column = week, row = weekday, exactly as the SVG lays it out.
+  const weeks = Math.ceil(s.heat.length / 7);
+  lines.push(
+    `      ${fg(MUTED, Array.from({ length: weeks }, (_, w) => ` ${String(w + 1).padStart(2)}`).join(""))}`,
+  );
+  WEEKDAYS.forEach((name, row) => {
+    const cells = Array.from({ length: weeks }, (_, w) => {
+      const entry = s.heat[w * 7 + row];
+      if (entry === undefined) return "   ";
+      // The marker sits in the gutter, not inside the cell: a foreground
+      // escape within a background run would end the background early.
+      const marker = s.campaign.day - 1 === w * 7 + row ? fg(FG, "▸") : " ";
+      return marker + heatCell(entry.attempts, "  ");
+    }).join("");
+    lines.push(`  ${fg(MUTED, cell(name, 4))}${cells}`);
+  });
+  lines.push("");
+  lines.push(
+    `  ${fg(MUTED, "less ")}${GREENS.map((_, level) => heatCell(level, "  ")).join("")}${fg(MUTED, " more")}` +
+      fg(MUTED, "   · one cell per day, colour = attempts, ▸ marks today"),
+  );
+
+  lines.push("");
+  lines.push(rule("weekly gates", cols));
+  if (s.gates.length === 0) {
+    lines.push(fg(MUTED, "  no gate has been scored yet"));
+  }
+  const gateW = Math.max(10, Math.min(24, cols - 30));
+  for (const g of s.gates) {
+    // Gates were stored as an integer percent before they were stored as a
+    // fraction; both are in the table, so normalise on read.
+    const rate = g.pass_rate === null ? null : g.pass_rate <= 1 ? g.pass_rate : g.pass_rate / 100;
+    const color = rate === null ? LINE : rate >= 0.7 ? GREEN : RED;
+    lines.push(
+      `  ${fg(FG, cell(`Week ${g.week}`, 9))}` +
+        `${bar(rate ?? 0, 1, gateW, color)}` +
+        `  ${fg(color, cell(rate === null ? "open" : `${Math.round(rate * 100)}%`, 5))}` +
+        `  ${fg(MUTED, g.issue === null ? "" : `#${g.issue}`)}${fg(MUTED, g.closed_on ? `  closed ${g.closed_on}` : "")}`,
+    );
+  }
+
+  return lines;
+}
+
+// ── panes ────────────────────────────────────────────────────────
+
+const PANES: { name: string; render: (s: Stats, cols: number) => string[] }[] = [
+  { name: "Overview", render: overview },
+  { name: "Ladder", render: ladder },
+  { name: "Concepts", render: concepts },
+  { name: "Activity", render: activity },
+];
+
+// ── non-interactive: every pane, once ────────────────────────────
+
+let stats: Stats;
+try {
+  stats = await fetchStats();
+} catch (err) {
+  console.error(`stats: ${err instanceof Error ? err.message : String(err)}`);
+  console.error(`stats: D1 is the only source of SRS truth — nothing to fall back to (SRS_API=${SRS_API}).`);
+  process.exit(1);
+}
+
+if (!process.stdout.isTTY) {
+  const cols = 80;
+  for (const pane of PANES) {
+    console.log(bold(pane.name.toUpperCase()));
+    for (const line of pane.render(stats, cols)) console.log(line);
+    console.log("");
+  }
+  process.exit(0);
+}
+
+// ── interactive ──────────────────────────────────────────────────
+
+let pane = 0;
+let offset = 0;
+let note = "";
+
+// Quitting resolves this; the finally below is what restores the terminal, so
+// a thrown render never leaves the shell in raw mode on the alternate screen.
+const { promise: quit, resolve: leave } = Promise.withResolvers<void>();
+const screen = openScreen(onKey, draw);
+
+function frame(): string[] {
+  const cols = screen.columns;
+  const tabs = PANES.map((p, i) =>
+    i === pane ? bold(fg(FG, ` ${p.name} `)) : fg(MUTED, ` ${p.name} `),
+  ).join(fg(LINE, "│"));
+  const clock = fg(MUTED, `day ${stats.campaign.day}/${stats.campaign.days} · streak ${stats.streak}  `);
+  const header = padTo(tabs, Math.max(0, cols - width(clock))) + clock;
+
+  // header + rule + body + footer fills the terminal exactly.
+  const body = PANES[pane]!.render(stats, cols);
+  const height = Math.max(1, screen.rows - 3);
+  offset = Math.max(0, Math.min(offset, Math.max(0, body.length - height)));
+  const scrolled =
+    body.length > height ? ` · ${offset + 1}-${Math.min(offset + height, body.length)}/${body.length}` : "";
+
+  return [
+    header,
+    fg(LINE, "─".repeat(cols)),
+    ...body.slice(offset, offset + height),
+    fg(MUTED, `Tab/←→ pane · ↑↓ scroll · r refresh · q quit${scrolled}`) + (note ? `  ${fg(YELLOW, note)}` : ""),
+  ];
+}
+
+function draw(): void {
+  screen.draw(frame());
+}
+
+function onKey(chunk: string): void {
+  switch (chunk) {
+    // Raw mode delivers Ctrl-C as a byte, not a signal, so it is handled here.
+    case "q":
+    case "\x03":
+    case "\x1b":
+      leave();
+      return;
+    case "\t":
+    case "\x1b[C":
+    case "l":
+      pane = (pane + 1) % PANES.length;
+      offset = 0;
+      break;
+    case "\x1b[Z":
+    case "\x1b[D":
+    case "h":
+      pane = (pane + PANES.length - 1) % PANES.length;
+      offset = 0;
+      break;
+    case "\x1b[A":
+    case "k":
+      offset = Math.max(0, offset - 1);
+      break;
+    case "\x1b[B":
+    case "j":
+      offset += 1;
+      break;
+    case "r":
+      note = "refreshing…";
+      draw();
+      void fetchStats()
+        .then((fresh) => {
+          stats = fresh;
+          note = "";
+        })
+        .catch((err: unknown) => {
+          note = `refresh failed: ${err instanceof Error ? err.message : String(err)}`;
+        })
+        .finally(draw);
+      return;
+    default:
+      if (chunk >= "1" && chunk <= String(PANES.length)) {
+        pane = Number(chunk) - 1;
+        offset = 0;
+        break;
+      }
+      return;
+  }
+  draw();
+}
+
+try {
+  draw();
+  await quit;
+} finally {
+  screen.close();
+}
@@ -4,18 +4,31 @@
 *
 * - New problems get a full page (frontmatter, badge, warning, examples,
 *   constraints, solution) in the gold-standard format.
- * - Already-ported pages only get their `## Solution` section regenerated,
- *   so hand-curated prose is never touched.
+ * - Already-ported pages only get their script-owned parts regenerated — the
+ *   `<Badge>` line and the `## Solution` section — so hand-curated prose is
+ *   never touched.
 * - When a problem is solved in both JavaScript and Python the solution
 *   renders as a <CodeGroup> (Python first); single-language solutions
 *   render as a plain fence.
 *
+ * Two classifications, two homes. The sidebar group is the campaign TOPIC
+ * from the README table (`tree-dfs/`, ordered by topic number via a
+ * script-owned `meta.ts`); the page badge is the data-structure CATEGORY the
+ * file sits under in `work/1/<Difficulty>/<Category>/`. A problem outside the
+ * plan (warm-ups) has no topic, so it groups under its category instead.
+ *
+ * Topic folders are real route segments (`/tree-dfs/invert-binary-tree`), not
+ * `(group)` folders, so a hand-written `index.mdx` in one is the topic's
+ * landing page at `/tree-dfs` and Blume links the sidebar group row to it.
+ * Sync never writes or moves an `index.mdx`, and never removes a folder that
+ * still holds one.
+ *
 * Only the first-solve bucket is read: work/3 and work/7 hold blind re-solves
 * of problems whose page already exists (see work.ts), and a fresh scaffold
 * there would otherwise wipe the page's `## Solution`.
 */
-import { mkdir, rename } from "node:fs/promises";
-import { basename, join } from "node:path";
+import { mkdir, readdir, rename, rmdir, unlink } from "node:fs/promises";
+import { basename, dirname, join } from "node:path";

 const ROOT = join(import.meta.dir, "..", "..");
 const WORK = join(ROOT, "work", "1");
@@ -158,24 +171,40 @@ function parseHeader(header: string): Header {

 // ── README topic map ─────────────────────────────────────────────

+/** A campaign topic: `<strong>NN. Name</strong>` in the README table. */
+interface Topic {
+  order?: number; // README topic number; absent for an unplanned problem's category group
+  name: string;
+}
+
 const TOPIC_ALIASES: Record<string, string> = {
  "Hash-Based Lookup": "Hash Table",
 };

-async function readmeTopics(): Promise<Map<number, string>> {
+/** LC number -> topic. A problem listed under two topics keeps the first. */
+async function readmeTopics(): Promise<Map<number, Topic>> {
  const md = await Bun.file(join(ROOT, "README.md")).text();
-  const map = new Map<number, string>();
-  let topic = "";
+  const map = new Map<number, Topic>();
+  let topic: Topic | null = null;
  for (const line of md.split("\n")) {
-    const t = line.match(/<strong>\d+\.\s*([^<]+)<\/strong>/);
-    if (t) topic = TOPIC_ALIASES[t[1]!.trim()] ?? t[1]!.trim();
+    const t = line.match(/<strong>(\d+)\.\s*([^<]+)<\/strong>/);
+    if (t) topic = { order: Number(t[1]), name: TOPIC_ALIASES[t[2]!.trim()] ?? t[2]!.trim() };
    if (!topic) continue;
    const n = line.match(/<td align="center">(\d+)<\/td>/);
-    if (n) map.set(Number(n[1]), topic);
+    if (n && !map.has(Number(n[1]))) map.set(Number(n[1]), topic);
  }
  return map;
 }

+/**
+ * Blume folder meta: exact topic label (the folder slug loses `/` and case) and
+ * curriculum order. An unplanned category group (warm-ups) has no README
+ * number and sorts at 0, ahead of topic 1, so the sidebar opens on the basics.
+ */
+function groupMeta(topic: Topic): string {
+  return `import { defineMeta } from "blume";\n\nexport default defineMeta({ title: ${JSON.stringify(topic.name)}, order: ${topic.order ?? 0} });\n`;
+}
+
 // ── inline-code formatting heuristics ────────────────────────────

 /** Identifiers worth backticking in prose, harvested from the code. */
@@ -277,7 +306,11 @@ function warningText(h: Header): string {
  );
 }

-function renderPage(sol: Solution, topic: string): string {
+function badgeLine(sol: Solution): string {
+  return `<Badge variant="accent">${sol.category}</Badge>`;
+}
+
+function renderPage(sol: Solution): string {
  const h = sol.header;
  const ids = identifiers(sol);
  const description = h.statement[0]?.replace(/\.\s*$/, "") ?? "";
@@ -292,7 +325,7 @@ function renderPage(sol: Solution, topic: string): string {
    `  badge: '${h.difficulty}'`,
    "---",
    "",
-    `<Badge variant="accent">${topic}</Badge>`,
+    badgeLine(sol),
    "",
  );

@@ -359,6 +392,24 @@ function ensureLabel(page: string, h: Header): string {
  return page.replace(/^sidebar:$/m, `sidebar:\n${label}`);
 }

+/**
+ * Offset just past the script-owned `## Solution` section: the next top-level
+ * heading that is not inside a code fence, or end of file. Sections a human
+ * appends after the solution (`## Explanation`, …) survive regeneration.
+ */
+function solutionEnd(page: string, from: number): number {
+  const lines = page.slice(from).split("\n");
+  let offset = from + lines[0]!.length + 1; // past the '## Solution' line itself
+  let fenced = false;
+  for (let i = 1; i < lines.length; i++) {
+    const line = lines[i]!;
+    if (line.startsWith("```")) fenced = !fenced;
+    else if (!fenced && line.startsWith("## ")) return offset;
+    offset += line.length + 1;
+  }
+  return page.length;
+}
+
 // Existing docs pages, keyed by leetcode number from the frontmatter title.
 const pages = new Map<number, string>(); // num -> path relative to DOCS
 for (const rel of (await Array.fromAsync(new Bun.Glob("**/*.mdx").scan(DOCS))).sort()) {
@@ -369,12 +420,19 @@ for (const rel of (await Array.fromAsync(new Bun.Glob("**/*.mdx").scan(DOCS))).s

 const report: [string, string, string][] = [];
 const ordered = [...solutions.values()].sort((a, b) => a.num - b.num);
+const groups = new Map<string, Topic>(); // folder -> topic, for meta.ts
+const vacated = new Set<string>(); // folders a page moved out of; removed below if empty

 for (const sol of ordered) {
  const langs = Object.keys(sol.code).sort().join("+");
  const workRef = `${sol.num}.${sol.slug} (${langs})`;
-  // (category) group folder + leetcode-number prefix: numeric sidebar order, both stripped from the URL.
-  const group = `(${sol.category.toLowerCase().replaceAll(" ", "-")})`;
+  // <topic>/ folder (a real route segment, so its index.mdx is the topic landing
+  // page) + leetcode-number prefix: numeric sidebar order, prefix stripped from the URL.
+  // An unplanned problem's category may share a folder with a numbered topic
+  // (`Hash Table` warm-ups under topic 1); the numbered one owns the meta.
+  const topic = topics.get(sol.num) ?? { name: sol.category };
+  const group = topic.name.toLowerCase().replace(/[^a-z0-9]+/g, "-").replace(/^-|-$/g, "");
+  if (groups.get(group)?.order === undefined) groups.set(group, topic);
  const name = `${group}/${sol.num}-${sol.slug}.mdx`;
  const existing = pages.get(sol.num);
  await mkdir(join(DOCS, group), { recursive: true });
@@ -386,10 +444,13 @@ for (const sol of ordered) {
      report.push([workRef, existing, "ERROR: no '## Solution' heading"]);
      continue;
    }
-    const next = ensureLabel(page.slice(0, at) + solutionSection(sol), sol.header);
+    const tail = page.slice(solutionEnd(page, at));
+    const body = page.slice(0, at) + solutionSection(sol) + (tail ? `\n${tail}` : "");
+    const next = ensureLabel(body, sol.header).replace(/^<Badge variant="accent">.*<\/Badge>$/m, badgeLine(sol));
    const actions: string[] = [];
    if (existing !== name) {
      await rename(join(DOCS, existing), join(DOCS, name));
+      vacated.add(dirname(existing));
      actions.push("moved");
    }
    if (next !== page) {
@@ -398,12 +459,30 @@ for (const sol of ordered) {
    }
    report.push([workRef, name, actions.join(" + ") || "skipped (in sync)"]);
  } else {
-    const topic = topics.get(sol.num) ?? sol.category;
-    await Bun.write(join(DOCS, name), renderPage(sol, topic));
+    await Bun.write(join(DOCS, name), renderPage(sol));
    report.push([workRef, name, "created"]);
  }
 }

+// Script-owned folder meta: one per live group, rewritten only when it differs.
+for (const [group, topic] of groups) {
+  const path = join(DOCS, group, "meta.ts");
+  const meta = groupMeta(topic);
+  if ((await Bun.file(path).exists()) && (await Bun.file(path).text()) === meta) continue;
+  await Bun.write(path, meta);
+  report.push([`topic ${topic.order ?? "-"}`, `${group}/meta.ts`, "written"]);
+}
+
+// A group every page left behind is dead: drop its meta and the folder itself.
+for (const dir of vacated) {
+  if (groups.has(dir)) continue;
+  const left = await readdir(join(DOCS, dir));
+  if (left.some((f) => f !== "meta.ts")) continue;
+  if (left.length) await unlink(join(DOCS, dir, "meta.ts"));
+  await rmdir(join(DOCS, dir));
+  report.push(["", dir, "removed (empty)"]);
+}
+
 const w0 = Math.max(...report.map((r) => r[0].length), 9);
 const w1 = Math.max(...report.map((r) => r[1].length), 9);
 console.log(`${"work file".padEnd(w0)}  ${"docs page".padEnd(w1)}  status`);
@@ -0,0 +1,214 @@
+/**
+ * Terminal drawing primitives for the stats dashboard: colour, block-glyph
+ * charts, and the alternate-screen plumbing.
+ *
+ * Hand-rolled for the same reason apps/api/src/charts.ts and png.ts are — the
+ * repo carries near-zero runtime dependencies, and everything a progress
+ * dashboard needs is a bar, a sparkline and a coloured cell. The palette is
+ * charts.ts's, so the terminal, the README SVGs and the digest email are the
+ * same picture in three renderers.
+ *
+ * Side-effect-free on import except for reading the colour environment once;
+ * nothing is written to the terminal until openScreen() is called.
+ */
+
+// ── palette (apps/api/src/charts.ts) ─────────────────────────────
+
+export const FG = "#fafafa"; // zinc-50 text
+export const MUTED = "#a1a1aa"; // zinc-400 secondary text
+export const LINE = "#27272a"; // zinc-800 structure / empty track
+export const GREEN = "#16a34a"; // green-600 (core / pass)
+export const BLUE = "#2563eb"; // blue-600 (optional)
+export const YELLOW = "#eab308"; // yellow-500 (attention)
+export const RED = "#f87171"; // red-400 (fail, on dark)
+
+/** GitHub dark-mode contribution hues; level 0 is the empty cell. */
+export const GREENS = ["#27272a", "#0e4429", "#006d32", "#26a641", "#39d353"];
+
+// ── colour ───────────────────────────────────────────────────────
+
+/**
+ * Honour NO_COLOR and pipes, and let FORCE_COLOR override both — the dashboard
+ * is legible without colour, and `bun run stats | less -R` should still be.
+ */
+export const COLOR =
+  process.env.FORCE_COLOR !== undefined
+    ? process.env.FORCE_COLOR !== "0"
+    : process.env.NO_COLOR === undefined && process.stdout.isTTY === true;
+
+const RESET = "\x1b[0m";
+
+function channels(hex: string): string {
+  return `${parseInt(hex.slice(1, 3), 16)};${parseInt(hex.slice(3, 5), 16)};${parseInt(hex.slice(5, 7), 16)}`;
+}
+
+export function fg(hex: string, text: string): string {
+  return COLOR ? `\x1b[38;2;${channels(hex)}m${text}${RESET}` : text;
+}
+
+export function bg(hex: string, text: string): string {
+  return COLOR ? `\x1b[48;2;${channels(hex)}m${text}${RESET}` : text;
+}
+
+export function bold(text: string): string {
+  return COLOR ? `\x1b[1m${text}${RESET}` : text;
+}
+
+const ANSI = /\x1b\[[0-9;]*m/g;
+
+/** Printable width — every glyph used here is one cell wide. */
+export function width(text: string): number {
+  return text.replace(ANSI, "").length;
+}
+
+/** Pad or cut a *plain* string to exactly n cells; colour it afterwards. */
+export function cell(text: string, n: number, align: "left" | "right" = "left"): string {
+  const clipped = text.length > n ? `${text.slice(0, Math.max(0, n - 1))}…` : text;
+  return align === "left" ? clipped.padEnd(n) : clipped.padStart(n);
+}
+
+/** Pad an already-coloured string to n cells. */
+export function padTo(text: string, n: number): string {
+  return text + " ".repeat(Math.max(0, n - width(text)));
+}
+
+// ── charts ───────────────────────────────────────────────────────
+
+const EIGHTHS = ["", "▏", "▎", "▍", "▌", "▋", "▊", "▉"];
+const SPARKS = ["▁", "▂", "▃", "▄", "▅", "▆", "▇", "█"];
+
+/**
+ * Filled and empty. The two differ by GLYPH as well as by colour, so a bar
+ * still reads under NO_COLOR or in a pipe — every chart here has to survive
+ * `bun run stats > notes.txt`.
+ */
+const FILL = "█";
+const TRACK = "░";
+
+/**
+ * One bar over a muted track, `cells` wide. Sub-cell remainders use the
+ * eighth-block glyphs, so one solved problem out of 161 still shows.
+ */
+export function bar(value: number, total: number, cells: number, color = GREEN): string {
+  const units = Math.round(Math.max(0, Math.min(1, total > 0 ? value / total : 0)) * cells * 8);
+  const filled = FILL.repeat(Math.floor(units / 8)) + EIGHTHS[units % 8]!;
+  return fg(color, filled) + fg(LINE, TRACK.repeat(Math.max(0, cells - filled.length)));
+}
+
+export interface Segment {
+  value: number;
+  color: string;
+  /** Defaults to the solid block; pass TRACK-ish glyphs for "not done yet". */
+  glyph?: string;
+}
+
+/**
+ * Segments laid end to end, `total` mapped to `cells`. Anything short of
+ * `total` is left BLANK, not tracked: bars sharing a scale must show their
+ * different lengths. A caller wanting a visible remainder passes it as a
+ * segment of its own.
+ *
+ * Non-zero segments are floored but never vanish; the overshoot that floor
+ * creates is taken back from the widest segment, which can spare it.
+ */
+export function stackedBar(segments: Segment[], total: number, cells: number): string {
+  if (total <= 0) return " ".repeat(cells);
+  const widths = segments.map((s) =>
+    s.value > 0 ? Math.max(1, Math.floor((s.value / total) * cells)) : 0,
+  );
+  let over = widths.reduce((a, b) => a + b, 0) - cells;
+  while (over > 0) {
+    const i = widths.indexOf(Math.max(...widths));
+    if (widths[i]! <= 1) break;
+    widths[i]!--;
+    over--;
+  }
+  const drawn = widths.map((n, i) => fg(segments[i]!.color, (segments[i]!.glyph ?? FILL).repeat(n)));
+  return drawn.join("") + " ".repeat(Math.max(0, cells - widths.reduce((a, b) => a + b, 0)));
+}
+
+/**
+ * One cell of the campaign heatmap. With colour it is the GREENS ramp the
+ * README and the digest use; without it, the same five levels as shading.
+ */
+export function heatCell(attempts: number, body: string): string {
+  const level = Math.min(attempts, GREENS.length - 1);
+  return COLOR ? bg(GREENS[level]!, body) : [" ", "·", "▒", "▓", "█"][level]!.repeat(body.length);
+}
+
+/** One glyph per value, scaled to the largest. All-zero renders as a floor. */
+export function sparkline(values: number[], color = GREEN): string {
+  const peak = Math.max(...values, 0);
+  const line = values
+    .map((v) => (peak === 0 ? SPARKS[0]! : SPARKS[Math.min(7, Math.round((v / peak) * 7))]!))
+    .join("");
+  return fg(color, line);
+}
+
+// ── layout ───────────────────────────────────────────────────────
+
+/** A section rule that fills the width: `── name ────────────`. */
+export function rule(title: string, cells: number): string {
+  const label = ` ${title} `;
+  return (
+    fg(LINE, "──") + fg(MUTED, label) + fg(LINE, "─".repeat(Math.max(0, cells - 2 - label.length)))
+  );
+}
+
+// ── screen ───────────────────────────────────────────────────────
+
+const ALT_ON = "\x1b[?1049h\x1b[?25l";
+const ALT_OFF = "\x1b[?25h\x1b[?1049l";
+
+export interface Screen {
+  readonly rows: number;
+  readonly columns: number;
+  /** Draw a frame, clipped to the terminal height. */
+  draw(lines: string[]): void;
+  close(): void;
+}
+
+/**
+ * Take over the terminal: alternate buffer, hidden cursor, raw keys. `onKey`
+ * receives the raw chunk (`"\t"`, `"\x1b[B"`, `"q"`, …) — decoding belongs to
+ * the app, which is the only thing that knows what a key means.
+ *
+ * The caller MUST close() in a finally: raw mode survives a thrown exception.
+ */
+export function openScreen(onKey: (chunk: string) => void, onResize: () => void): Screen {
+  const out = process.stdout;
+  const input = process.stdin;
+
+  out.write(ALT_ON);
+  input.setRawMode?.(true);
+  input.setEncoding("utf8");
+  input.resume();
+  input.on("data", onKey);
+  out.on("resize", onResize);
+
+  let closed = false;
+  return {
+    get rows() {
+      return out.rows ?? 24;
+    },
+    get columns() {
+      return out.columns ?? 80;
+    },
+    draw(lines: string[]): void {
+      const visible = lines.slice(0, this.rows);
+      // Home, then erase each line as it is rewritten plus everything below the
+      // last one. No full clear, so a redraw does not flash, and no trailing
+      // newline, which would scroll the alternate screen by a row per frame.
+      out.write(`\x1b[H${visible.map((l) => `${l}\x1b[K`).join("\r\n")}\x1b[J`);
+    },
+    close(): void {
+      if (closed) return;
+      closed = true;
+      input.off("data", onKey);
+      out.off("resize", onResize);
+      input.setRawMode?.(false);
+      input.pause();
+      out.write(ALT_OFF);
+    },
+  };
+}
@@ -0,0 +1,173 @@
+#!/usr/bin/env bun
+/**
+ * Find the NeetCode explanation video for every docs page that has none.
+ *
+ * A finished solution page ends with `## Explanation` holding one
+ * `<YouTube id="…" title="…" />` — NeetCode's walkthrough of the problem.
+ * `sync.ts` never writes that section (it owns only `## Solution`), so pages
+ * accumulate without one. This script lists every numbered page lacking a
+ * `<YouTube` embed and looks each up on NeetCode's two channels, main channel
+ * first, accepting only a title that names the page's exact LC number
+ * (`… - Leetcode 226 - …`) and is not a #short. No YouTube API key: the
+ * channel search page embeds its results as `ytInitialData` JSON, which is
+ * all we parse. Ranking is the channel's own — the first exact-number match
+ * wins.
+ *
+ *   bun apps/cli/videos.ts           # report: page → video (or "no match")
+ *   bun apps/cli/videos.ts --write   # also append `## Explanation` + embed
+ *
+ * --write appends at end of file (below `## Solution`, where sync.ts leaves
+ * human sections alone) and adds the `## Explanation` heading only when the
+ * page lacks it. A page with no match is left untouched, so re-runs are
+ * no-ops once every page has its embed.
+ */
+import { join } from "node:path";
+
+import { printTable } from "./report.ts";
+import { ROOT } from "./work.ts";
+
+const DOCS = join(ROOT, "apps", "docs", "content");
+const WRITE = process.argv.includes("--write");
+
+/** Channel handles searched in order; the first exact match wins. */
+const CHANNELS = ["NeetCode", "NeetCodeIO"] as const;
+
+interface Video {
+  id: string;
+  title: string;
+  channel: string;
+}
+
+// ── YouTube channel search (no API key) ──────────────────────────
+
+const HEADERS = { "accept-language": "en-US,en;q=0.9", "user-agent": "Mozilla/5.0" };
+
+/**
+ * The `var ytInitialData = {…};</script>` blob of a YouTube page, parsed.
+ * YouTube intermittently answers 200 with a page that lacks the blob or cuts
+ * it short, so a bad page is retried a few times before it is an error.
+ */
+async function initialData(url: string): Promise<unknown> {
+  let reason = "";
+  for (let attempt = 0; attempt < 3; attempt++) {
+    if (attempt) await Bun.sleep(500 * attempt);
+    const res = await fetch(url, { headers: HEADERS });
+    if (!res.ok) {
+      reason = `status ${res.status}`;
+      continue;
+    }
+    const html = await res.text();
+    // Slice rather than regex: the blob is ~1 MB, and a lazy `.*?` across it
+    // is what made the first cut of this script fail at random.
+    const open = html.search(/ytInitialData\s*=\s*\{/);
+    const close = open === -1 ? -1 : html.indexOf("</script>", open);
+    if (close === -1) {
+      reason = `no ytInitialData in ${html.length}-byte page`;
+      continue;
+    }
+    try {
+      return JSON.parse(html.slice(html.indexOf("{", open), close).replace(/;\s*$/, ""));
+    } catch {
+      reason = "truncated ytInitialData";
+    }
+  }
+  throw new Error(`GET ${url} -> ${reason}`);
+}
+
+/** Every videoRenderer in a channel search page, in the page's own order. */
+async function channelSearch(channel: string, query: string): Promise<Video[]> {
+  const url = `https://www.youtube.com/@${channel}/search?query=${encodeURIComponent(query)}`;
+  const data = await initialData(url);
+
+  const videos: Video[] = [];
+  const walk = (node: unknown): void => {
+    if (!node || typeof node !== "object") return;
+    if ("videoRenderer" in node) {
+      const v = node.videoRenderer;
+      if (v && typeof v === "object" && "videoId" in v && typeof v.videoId === "string") {
+        const runs =
+          "title" in v && v.title && typeof v.title === "object" && "runs" in v.title
+            ? v.title.runs
+            : undefined;
+        const title = Array.isArray(runs)
+          ? runs.map((r) => (r && typeof r === "object" && "text" in r ? String(r.text) : "")).join("")
+          : "";
+        videos.push({ id: v.videoId, title, channel });
+      }
+      return;
+    }
+    for (const child of Object.values(node)) walk(child);
+  };
+  walk(data);
+  return videos;
+}
+
+/** The LC number a NeetCode title names (`… - Leetcode 226 - …`), if any. */
+function titledNumber(title: string): number | undefined {
+  const m = title.match(/\bleetcode\s*#?\s*(\d+)\b/i);
+  return m ? Number(m[1]) : undefined;
+}
+
+async function findVideo(num: number): Promise<Video | undefined> {
+  for (const channel of CHANNELS) {
+    const hit = (await channelSearch(channel, `leetcode ${num}`)).find(
+      (v) => titledNumber(v.title) === num && !/#shorts\b/i.test(v.title),
+    );
+    if (hit) return hit;
+  }
+  return undefined;
+}
+
+// ── docs pages ───────────────────────────────────────────────────
+
+interface Page {
+  num: number;
+  rel: string;
+  text: string;
+}
+
+/** Numbered solution pages without a `<YouTube` embed, by LC number. */
+async function pagesMissingVideo(): Promise<Page[]> {
+  const pages: Page[] = [];
+  for (const rel of await Array.fromAsync(new Bun.Glob("**/*.mdx").scan(DOCS))) {
+    const text = await Bun.file(join(DOCS, rel)).text();
+    const t = text.match(/^title: '(\d+)\./m);
+    if (t && !text.includes("<YouTube")) pages.push({ num: Number(t[1]), rel, text });
+  }
+  return pages.sort((a, b) => a.num - b.num);
+}
+
+function withEmbed(page: Page, video: Video): string {
+  const embed = `<YouTube id="${video.id}" title="${video.title.replaceAll('"', "'")}" />`;
+  const heading = /^## Explanation\s*$/m.test(page.text) ? "" : "## Explanation\n\n";
+  return `${page.text.replace(/\s*$/, "\n")}\n${heading}${embed}\n`;
+}
+
+// ── main ─────────────────────────────────────────────────────────
+
+const missing = await pagesMissingVideo();
+if (missing.length === 0) {
+  console.log("every solution page has a video");
+  process.exit(0);
+}
+
+const rows: string[][] = [["docs page", "video", "title", WRITE ? "status" : ""]];
+let found = 0;
+for (const page of missing) {
+  const video = await findVideo(page.num);
+  if (!video) {
+    rows.push([page.rel, "—", "no NeetCode match", WRITE ? "skipped" : ""]);
+    continue;
+  }
+  found++;
+  let status = "";
+  if (WRITE) {
+    await Bun.write(join(DOCS, page.rel), withEmbed(page, video));
+    status = "written";
+  }
+  rows.push([page.rel, `https://youtu.be/${video.id}`, `${video.title} (${video.channel})`, status]);
+}
+
+if (!WRITE) for (const row of rows) row.pop();
+printTable(rows);
+console.log(`\n${found}/${missing.length} page(s) matched${WRITE ? `, ${found} written` : ""}`);
@@ -93,6 +93,25 @@ export function scanBucket(bucket: Bucket): Map<number, WorkFile[]> {
  return files;
 }

+/**
+ * Is this file a real solution rather than an empty scaffold? The same call
+ * close-solved.ts makes before it closes an issue — a stub under work/1 is a
+ * problem still unsolved, and owes no review.
+ */
+export function isSolution(file: WorkFile): boolean {
+  const source = readFileSync(join(WORK, file.rel), "utf8");
+  return isImplemented(source, file.rel.endsWith(".py") ? "py" : "js");
+}
+
+/** Problems in a bucket with a solution on disk; scaffolds do not count. */
+export function solvedInBucket(bucket: Bucket): number {
+  let solved = 0;
+  for (const files of scanBucket(bucket).values()) {
+    if (files.some(isSolution)) solved++;
+  }
+  return solved;
+}
+
 /**
 * LC number -> ET date of the commit that first added a work/ file for it.
 *
@@ -153,14 +172,12 @@ export function reviewQueues(today: string): Map<Window, Owed[]> {
  const dates = firstSolved();

  // An empty scaffold under work/1 is a problem still unsolved, not one owing
-  // a review — the same call close-solved.ts makes before it closes an issue.
+  // a review.
  const solved: { file: WorkFile; date: string }[] = [];
  for (const files of scanBucket(1).values()) {
    const date = dates.get(files[0]!.lc);
    if (date === undefined) continue; // uncommitted: you only just solved it
-    const file = files.find((f) =>
-      isImplemented(readFileSync(join(WORK, f.rel), "utf8"), f.rel.endsWith(".py") ? "py" : "js"),
-    );
+    const file = files.find(isSolution);
    if (file) solved.push({ file, date });
  }

@@ -10,12 +10,4 @@ export default defineConfig({
  content: {
    root: "content",
  },
-  navigation: {
-    tabs: [
-      {
-        label: "Reference",
-        path: "/reference",
-      },
-    ],
-  },
 });
@@ -0,0 +1,13 @@
+/**
+ * Project MDX components, available on every page with no import. Static
+ * `.astro` files ship no framework JS (a custom element carries any
+ * behaviour); interactive React lives in `islands/` instead.
+ */
+import { defineComponents } from "blume";
+
+export default defineComponents({
+  mdx: {
+    Carousel: "./components/Carousel.astro",
+    CarouselImage: "./components/CarouselImage.astro",
+  },
+});
@@ -0,0 +1,170 @@
+---
+/**
+ * A horizontal carousel of <CarouselImage> slides, built the way Blume's own
+ * content components are: static markup, a custom element for the behaviour,
+ * no framework. The track is a CSS scroll-snap strip (so it also swipes on
+ * touch and the browser does the animation); prev/next sit outside it and
+ * the play toggle sits below with a "n / N" counter.
+ *
+ * `time` is seconds per slide while playing (default 1.25); `autoplay` starts
+ * playing on load. Play wraps around, pauses whenever the tab is hidden, and
+ * never starts on its own for a reader who prefers reduced motion — the
+ * button still works for them. Manual navigation (buttons, arrow keys, swipe)
+ * does not stop playback; it only restarts the interval so a step is never
+ * doubled.
+ */
+import Icon from "blume/components/Icon.astro";
+
+interface Props {
+  /** Seconds each slide stays while playing. */
+  time?: number;
+  autoplay?: boolean;
+}
+
+const { time = 1.25, autoplay = false } = Astro.props;
+
+const buttonClass =
+  "inline-flex size-9 shrink-0 cursor-pointer items-center justify-center rounded-full border border-border bg-background text-muted-foreground transition-colors hover:border-foreground hover:text-foreground disabled:cursor-default disabled:opacity-40";
+---
+
+<blume-carousel
+  class="not-prose my-6 block"
+  data-time={String(time)}
+  data-autoplay={autoplay ? "" : undefined}
+  role="region"
+  aria-roledescription="carousel"
+>
+  <div class="flex items-center gap-3">
+    <button class={buttonClass} data-prev type="button" aria-label="Previous slide">
+      <Icon class="rtl:-scale-x-100" name="chevron-left" size={18} />
+    </button>
+    <div
+      class="flex min-w-0 flex-1 snap-x snap-mandatory overflow-x-auto scroll-smooth rounded-blume [scrollbar-width:none] [&::-webkit-scrollbar]:hidden"
+      data-track
+      tabindex="0"
+      aria-live="polite"
+    >
+      <slot />
+    </div>
+    <button class={buttonClass} data-next type="button" aria-label="Next slide">
+      <Icon class="rtl:-scale-x-100" name="chevron-right" size={18} />
+    </button>
+  </div>
+  <div class="mt-3 flex items-center justify-center gap-3">
+    <button class={buttonClass} data-play type="button" aria-label="Play" aria-pressed="false">
+      <Icon class="[[aria-pressed=true]_&]:hidden" name="play" size={16} />
+      <Icon class="[[aria-pressed=false]_&]:hidden" name="pause" size={16} />
+    </button>
+    <span class="text-muted-foreground text-xs tabular-nums leading-5" data-count></span>
+  </div>
+</blume-carousel>
+
+<script>
+  class BlumeCarousel extends HTMLElement {
+    #track!: HTMLElement;
+    #play!: HTMLButtonElement;
+    #count!: HTMLElement;
+    #timer: number | null = null;
+    #index = 0;
+
+    connectedCallback() {
+      this.#track = this.querySelector("[data-track]")!;
+      this.#play = this.querySelector("[data-play]")!;
+      this.#count = this.querySelector("[data-count]")!;
+      const prev = this.querySelector<HTMLButtonElement>("[data-prev]")!;
+      const next = this.querySelector<HTMLButtonElement>("[data-next]")!;
+
+      const slides = this.#slides().length;
+      if (slides < 2) {
+        for (const b of [prev, next, this.#play]) b.disabled = true;
+      }
+      this.#render();
+
+      prev.addEventListener("click", () => this.#step(-1));
+      next.addEventListener("click", () => this.#step(1));
+      this.#play.addEventListener("click", () => (this.#timer === null ? this.#start() : this.#stop()));
+      this.addEventListener("keydown", (event) => {
+        if (event.key === "ArrowLeft") this.#step(-1);
+        else if (event.key === "ArrowRight") this.#step(1);
+        else return;
+        event.preventDefault();
+      });
+      // Swipes and trackpad scrolls move the track without #step; follow them
+      // once the scroll settles. Settle-only, not per event: a long smooth
+      // scroll from #step passes over intermediate slides and must not be
+      // read back as the reader's position mid-flight.
+      let settle: number | undefined;
+      this.#track.addEventListener(
+        "scroll",
+        () => {
+          window.clearTimeout(settle);
+          settle = window.setTimeout(() => this.#syncFromScroll(), 150);
+        },
+        { passive: true }
+      );
+      document.addEventListener("visibilitychange", this.#onVisibility);
+
+      const reduced = matchMedia("(prefers-reduced-motion: reduce)").matches;
+      if (this.hasAttribute("data-autoplay") && slides > 1 && !reduced) this.#start();
+    }
+
+    disconnectedCallback() {
+      this.#stop();
+      document.removeEventListener("visibilitychange", this.#onVisibility);
+    }
+
+    #slides(): HTMLElement[] {
+      return [...this.#track.querySelectorAll<HTMLElement>(":scope > blume-carousel-image")];
+    }
+
+    #step(delta: number) {
+      const slides = this.#slides();
+      this.#index = (this.#index + delta + slides.length) % slides.length;
+      slides[this.#index]!.scrollIntoView({ block: "nearest", inline: "start", behavior: "smooth" });
+      this.#render();
+      if (this.#timer !== null) this.#start(); // restart the interval from this slide
+    }
+
+    #syncFromScroll() {
+      const at = Math.round(this.#track.scrollLeft / this.#track.clientWidth);
+      if (at !== this.#index && Number.isFinite(at)) {
+        this.#index = Math.min(Math.max(at, 0), this.#slides().length - 1);
+        this.#render();
+      }
+    }
+
+    #start() {
+      this.#stop();
+      const ms = Number(this.dataset.time) * 1000;
+      this.#timer = window.setInterval(() => this.#step(1), ms > 0 ? ms : 1250);
+      this.#play.setAttribute("aria-pressed", "true");
+      this.#play.setAttribute("aria-label", "Pause");
+    }
+
+    #stop() {
+      if (this.#timer !== null) window.clearInterval(this.#timer);
+      this.#timer = null;
+      this.#play.setAttribute("aria-pressed", "false");
+      this.#play.setAttribute("aria-label", "Play");
+    }
+
+    // A background tab keeps its timers but not its layout; resume on return.
+    #wasPlaying = false;
+    #onVisibility = () => {
+      if (document.hidden) {
+        this.#wasPlaying = this.#timer !== null;
+        this.#stop();
+      } else if (this.#wasPlaying) this.#start();
+    };
+
+    #render() {
+      const slides = this.#slides();
+      this.#count.textContent = `${this.#index + 1} / ${slides.length}`;
+      slides.forEach((s, i) => s.setAttribute("aria-hidden", i === this.#index ? "false" : "true"));
+    }
+  }
+
+  if (!customElements.get("blume-carousel")) {
+    customElements.define("blume-carousel", BlumeCarousel);
+  }
+</script>
@@ -0,0 +1,45 @@
+---
+/**
+ * One slide of a <Carousel>: Blume's own <Frame> around an <img>, so a slide
+ * is pixel-identical to a framed image elsewhere on the site — `alt` doubles
+ * as the frame's hint line and `caption` is Markdown, exactly as in Frame.
+ *
+ * `src` accepts what MDX authors already write for images: a URL, a
+ * root-relative `/file.png` (served from public/, prefixed with the deploy
+ * base), or the page-relative `../../public/file.png` form. A relative path
+ * that does not pass through public/ has nothing to serve it, so it is a
+ * build error rather than a silent 404.
+ */
+import Frame from "blume/components/content/Frame.astro";
+import { withBase } from "blume/components/islands/base-path.ts";
+
+interface Props {
+  src: string;
+  alt?: string;
+  caption?: string;
+}
+
+const { src, alt, caption } = Astro.props;
+
+const resolveSrc = (value: string): string => {
+  if (URL.canParse(value)) return value;
+  if (value.startsWith("/")) return withBase(value);
+  const at = value.indexOf("public/");
+  if (at === -1) {
+    throw new Error(
+      `<CarouselImage src="${value}">: a relative src must point into public/ (or use /file.png)`
+    );
+  }
+  return withBase(`/${value.slice(at + "public/".length)}`);
+};
+---
+
+<blume-carousel-image
+  class="block w-full shrink-0 snap-center [&_blume-frame]:my-0"
+  role="group"
+  aria-roledescription="slide"
+>
+  <Frame caption={caption} hint={alt}>
+    <img alt={alt ?? ""} src={resolveSrc(src)} loading="lazy" draggable="false" />
+  </Frame>
+</blume-carousel-image>
@@ -26,6 +26,20 @@ sidebar:
 - `2 <= nums.length <= 500`
 - `0 <= nums[i] <= 100`

+## Approach
+
+```mermaid
+flowchart TD
+  S(["smallerNumbersThanCurrent(nums)"]) --> F["freq: count every value in nums"]
+  F --> O["sorted = keys of freq, ascending"]
+  O --> I["count = 0, smaller = {}"]
+  I --> L{"more num in sorted?"}
+  L -- yes --> A["smaller[num] = count — everything already passed is smaller"]
+  A --> B["count += freq[num]"]
+  B --> L
+  L -- no --> M(["return nums.map(n => smaller[n])"])
+```
+
 ## Solution

 ```js
@@ -37,6 +37,18 @@ startValue = 4 | startValue = 5 | nums
 - `1 <= nums.length <= 100`
 - `-100 <= nums[i] <= 100`

+## Approach
+
+```mermaid
+flowchart TD
+  S(["minStartValue(nums)"]) --> I["prefix = [nums[0]]"]
+  I --> L{"more i in 1..n-1?"}
+  L -- yes --> P["prefix.push(prefix[i-1] + nums[i])"]
+  P --> L
+  L -- no --> M["min = smallest running total"]
+  M --> R(["return max(1, 1 - min) — lift the dip to 1"])
+```
+
 ## Solution

 ```js
@@ -23,6 +23,19 @@ sidebar:
 - `1 <= arr.length <= 1000`
 - `0 <= arr[i] <= 1000`

+## Approach
+
+```mermaid
+flowchart TD
+  S(["countElements(arr)"]) --> I["arrSet = Set(arr), count = 0"]
+  I --> L{"more n in arr?"}
+  L -- no --> E(["return count"])
+  L -- yes --> H{"arrSet has n + 1?"}
+  H -- yes --> C["count++"]
+  C --> L
+  H -- no --> L
+```
+
 ## Solution

 ```js
@@ -27,6 +27,17 @@ sidebar:
 - `1 <= nums.length <= 1000`
 - `-10^6 <= nums[i] <= 10^6`

+## Approach
+
+```mermaid
+flowchart TD
+  S(["runningSum(nums)"]) --> I["prefix = [nums[0]]"]
+  I --> L{"more i in 1..n-1?"}
+  L -- yes --> P["prefix.push(prefix[i-1] + nums[i])"]
+  P --> L
+  L -- no --> E(["return prefix"])
+```
+
 ## Solution

 ```js
@@ -27,6 +27,18 @@ sidebar:
 - `1 <= nums.length <= 100`
 - `-100 <= nums[i] <= 100`

+## Approach
+
+```mermaid
+flowchart TD
+  S(["frequencySort(nums)"]) --> F["freq: count every value in nums"]
+  F --> C["sort nums with a two-key comparator"]
+  C --> K1["primary: freq[a] - freq[b] — rarer first"]
+  C --> K2["tie-break: b - a — larger value first"]
+  K1 --> R(["return nums"])
+  K2 --> R
+```
+
 ## Solution

 ```js
@@ -40,3 +52,7 @@ var frequencySort = function (nums) {
  return nums.sort((a, b) => freq[a] - freq[b] || b - a);
 };
 ```
+
+## Explanation
+
+<YouTube id="Evq1SfUbhBg" title="Sort Array by Increasing Frequency - Leetcode 1636 - Python" />
@@ -34,6 +34,18 @@ Could you implement a solution using only O(1) extra space complexity and O(n) r
 - `0 <= nums[i] <= n`
 - All the numbers of `nums` are unique.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["missingNumber(nums)"]) --> I["numSet = Set(nums), expectedCount = n + 1"]
+  I --> L{"more i in 0..expectedCount-1?"}
+  L -- yes --> Q{"numSet has i?"}
+  Q -- no --> R(["return i"])
+  Q -- yes --> L
+  L -- no --> E(["return -1"])
+```
+
 ## Solution

 ```js
@@ -53,3 +65,7 @@ var missingNumber = function(nums) {
  return -1;
 };
 ```
+
+## Explanation
+
+<YouTube id="WnPLSRLSANE" title="Missing Number - Blind 75 - Leetcode 268 - Python" />
@@ -0,0 +1,73 @@
+---
+title: '271. Encode and Decode Strings'
+description: Design an algorithm to encode a list of strings to a string. The encoded string is then sent over the network and is decoded back to the original list of strings
+sidebar:
+  label: 'Encode and Decode Strings'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Array</Badge>
+
+::::warning
+Could you write a generalized algorithm to work on any possible set of characters?
+::::
+
+### Example 1:
+- Input: `dummy_input = ["Hello","World"]`
+- Output: `["Hello","World"]`
+- Explanation: Machine `1`: Codec encoder = new Codec(); String msg = encoder.encode(strs); Machine `1` ---msg---> Machine `2` Machine `2`: Codec decoder = new Codec(); `String[] strs` = decoder.decode(msg);
+
+### Example 2:
+- Input: `dummy_input = [""]`
+- Output: `[""]`
+
+### Constraints:
+
+- `1 <= strs.length <= 200`
+- `0 <= strs[i].length <= 200`
+- `strs[i]` contains any possible characters out of 256 valid ASCII characters.
+
+## Approach
+
+::::warning
+The diagram traces the code below literally. That code does not solve the problem: both `encode` and `decode` have only a docstring for a body, so each returns `None` and `codec.decode(codec.encode(strs))` yields `None` instead of `strs`.
+::::
+
+```mermaid
+flowchart TD
+  subgraph state["state — Codec holds no fields"]
+    X["Codec() — nothing is stored between encode and decode"]
+  end
+  subgraph encode["encode(strs)"]
+    E1["docstring only — no statement runs"] --> E2(["return None"])
+  end
+  subgraph decode["decode(s)"]
+    D1["docstring only — no statement runs"] --> D2(["return None"])
+  end
+  state --> encode
+  state --> decode
+```
+
+## Solution
+
+```py
+class Codec:
+    def encode(self, strs: List[str]) -> str:
+        """Encodes a list of strings to a single string.
+        """
+        
+
+    def decode(self, s: str) -> List[str]:
+        """Decodes a single string to a list of strings.
+        """
+        
+
+
+# Your Codec object will be instantiated and called as such:
+# codec = Codec()
+# codec.decode(codec.encode(strs))
+```
+
+## Explanation
+
+<YouTube id="B1k_sxOSgv8" title="Encode and Decode Strings - Leetcode 271 - Python" />
@@ -0,0 +1,3 @@
+import { defineMeta } from "blume";
+
+export default defineMeta({ title: "Array", order: 0 });
@@ -0,0 +1,53 @@
+---
+title: '39. Combination Sum'
+description: Given an array of distinct integers candidates and a target integer target, return a list of all unique combinations of candidates where the chosen numbers sum to target. You may return the combinations in any order
+sidebar:
+  label: 'Combination Sum'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Array</Badge>
+
+### Example 1:
+- Input: `candidates = [2,3,6,7], target = 7`
+- Output: `[[2,2,3],[7]]`
+- Explanation: `2` and `3` are `candidates`, and `2 + 2 + 3 = 7`. Note that `2` can be used multiple times. `7` is a candidate, and `7 = 7`. These are the only two combinations.
+
+### Example 2:
+- Input: `candidates = [2,3,5], target = 8`
+- Output: `[[2,2,2,2],[2,3,3],[3,5]]`
+
+### Example 3:
+- Input: `candidates = [2], target = 1`
+- Output: `[]`
+
+### Constraints:
+
+- `1 <= candidates.length <= 30`
+- `2 <= candidates[i] <= 40`
+- All elements of `candidates` are distinct.
+- `1 <= target <= 40`
+
+## Solution
+
+```py
+class Solution:
+    def combinationSum(self, candidates: list[int], target: int) -> list[list[int]]:
+        candidates.sort()
+        result, path = [], []
+
+        def backtrack(start, remaining):
+            if remaining == 0:
+                result.append(path[:])
+                return
+            for i in range(start, len(candidates)):
+                if candidates[i] > remaining:
+                    break
+
+                path.append(candidates[i])
+                backtrack(i, remaining - candidates[i])
+                path.pop()
+
+        backtrack(0, target)
+        return result
+```
@@ -0,0 +1,52 @@
+---
+title: '46. Permutations'
+description: Given an array nums of distinct integers, return all the possible permutations. You can return the answer in any order
+sidebar:
+  label: 'Permutations'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Array</Badge>
+
+### Example 1:
+- Input: `nums = [1,2,3]`
+- Output: `[[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]`
+
+### Example 2:
+- Input: `nums = [0,1]`
+- Output: `[[0,1],[1,0]]`
+
+### Example 3:
+- Input: `nums = [1]`
+- Output: `[[1]]`
+
+### Constraints:
+
+- `1 <= nums.length <= 6`
+- `-10 <= nums[i] <= 10`
+- All the integers of `nums` are unique.
+
+## Solution
+
+```py
+class Solution:
+    def permute(self, nums: list[int]) -> list[list[int]]:
+        result, path = [], []
+        used = [False] * len(nums)
+
+        def backtrack():
+            if len(path) == len(nums):
+                result.append(path[:])
+                return
+            for i in range(len(nums)):
+                if used[i]:
+                    continue
+                used[i] = True
+                path.append(nums[i])
+                backtrack()
+                path.pop()
+                used[i] = False
+
+        backtrack()
+        return result
+```
@@ -0,0 +1,41 @@
+---
+title: '78. Subsets'
+description: Given an integer array nums of unique elements, return all possible subsets (the power set)
+sidebar:
+  label: 'Subsets'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Array</Badge>
+
+### Example 1:
+- Input: `nums = [1,2,3]`
+- Output: `[[],[1],[2],[1,2],[3],[1,3],[2,3],[1,2,3]]`
+
+### Example 2:
+- Input: `nums = [0]`
+- Output: `[[],[0]]`
+
+### Constraints:
+
+- `1 <= nums.length <= 10`
+- `-10 <= nums[i] <= 10`
+- All the numbers of `nums` are unique.
+
+## Solution
+
+```py
+class Solution:
+    def subsets(self, nums: list[int]) -> list[list[int]]:
+        res = []
+
+        def backtrack(i, path):
+            res.append(path[:])
+            for num_idx in range(i, len(nums)):
+                path.append(nums[num_idx])
+                backtrack(num_idx + 1, path)
+                path.pop()
+
+        backtrack(0, [])
+        return res
+```
@@ -0,0 +1,163 @@
+---
+title: Backtracking
+description: 'Backtracking is a depth-first search technique that builds a solution step by step and abandons (undoes) a partial choice as soon as it can’t lead to a valid complete solution.'
+hidden: true
+---
+
+## What it is
+
+Backtracking explores possible paths to a solution one step at a time. When a path reaches a dead end, you undo the last step and try a different option. Think of every problem as a decision tree. The root is an empty answer. Each level is a decision. Each branch is one option.
+
+## When to use it
+
+Use backtracking when you must generate **all** valid solutions or evaluate **all** combinations. Do not use it when you need only one solution or the optimal solution. For those, use greedy or dynamic programming. Backtracking has exponential time complexity.
+
+## The four components
+
+Every backtracking problem has the same parts:
+
+1. **Base case** — when to save the current path and stop.
+2. **Choices** — the options at each step.
+3. **Constraints** — the rules that reject an invalid path early.
+4. **Backtrack step** — undo the last choice and try the next one.
+
+## The template
+
+```python
+def solve(data):
+    result = []
+    path = []
+
+    def backtrack(index):
+        # 1. Base case
+        if is_complete(index, path):
+            result.append(path[:])  # copy — lists are passed by reference
+            return
+
+        # 2. Choices
+        for choice in get_choices(index, data):
+            # 3. Constraints
+            if not is_valid(choice, path):
+                continue
+
+            path.append(choice)    # make the choice
+            backtrack(index + 1)   # explore
+            path.pop()             # 4. Backtrack
+
+    backtrack(0)
+    return result
+```
+
+## Steps to apply the template
+
+1. Draw the decision tree for a small input. Name each level and each branch.
+2. Write the base case. Ask: when is the path complete?
+3. Write the choices. Ask: what can I add at this level?
+4. Write the constraints. Ask: what makes a path invalid? Reject it before recursion.
+5. Add the backtrack step. Pop what you appended.
+6. Copy the path when you save it. Never append the same list reference.
+
+## Four problems, one template
+
+### Subsets
+
+Choice: include or skip each number. No constraint. Base case: index equals list length.
+
+```python
+def subsets(nums):
+    result, path = [], []
+
+    def backtrack(i):
+        if i == len(nums):
+            result.append(path[:])
+            return
+        path.append(nums[i])   # include
+        backtrack(i + 1)
+        path.pop()
+        backtrack(i + 1)       # skip
+
+    backtrack(0)
+    return result
+```
+
+### Permutations
+
+Same choices as subsets. Constraint: do not reuse a number already in the path. Base case: path length equals list length.
+
+```python
+def permute(nums):
+    result, path = [], []
+    used = [False] * len(nums)
+
+    def backtrack():
+        if len(path) == len(nums):
+            result.append(path[:])
+            return
+        for i in range(len(nums)):
+            if used[i]:
+                continue
+            used[i] = True
+            path.append(nums[i])
+            backtrack()
+            path.pop()
+            used[i] = False
+
+    backtrack()
+    return result
+```
+
+### Combinations
+
+Same as subsets. Change one line: save the path only when its length equals `k`.
+
+```python
+def combine(n, k):
+    result, path = [], []
+
+    def backtrack(start):
+        if len(path) == k:
+            result.append(path[:])
+            return
+        for i in range(start, n + 1):
+            path.append(i)
+            backtrack(i + 1)
+            path.pop()
+
+    backtrack(1)
+    return result
+```
+
+### N-Queens
+
+Choice: which column to place the queen in the current row. Constraint: no shared column or diagonal. Base case: `n` queens placed. Backtrack: remove the last queen.
+
+```python
+def solve_n_queens(n):
+    result, cols = [], []
+    used_cols, diag1, diag2 = set(), set(), set()
+
+    def backtrack(row):
+        if row == n:
+            result.append(cols[:])
+            return
+        for col in range(n):
+            if col in used_cols or row - col in diag1 or row + col in diag2:
+                continue
+            cols.append(col)
+            used_cols.add(col); diag1.add(row - col); diag2.add(row + col)
+            backtrack(row + 1)
+            cols.pop()
+            used_cols.remove(col); diag1.remove(row - col); diag2.remove(row + col)
+
+    backtrack(0)
+    return result
+```
+
+## Common mistakes
+
+- You append the `path` list itself instead of a copy. The result fills with empty lists.
+- You forget to pop. The path grows and never resets.
+- You check constraints after recursion instead of before. You waste time on dead paths.
+- You use backtracking when you need only one answer. Use greedy or DP instead.
+
+
@@ -0,0 +1,3 @@
+import { defineMeta } from "blume";
+
+export default defineMeta({ title: "Backtracking", order: 20 });
@@ -0,0 +1,75 @@
+---
+title: '230. Kth Smallest Element in a BST'
+description: Given the root of a binary search tree, and an integer k, return the k^th smallest value (1-indexed) of all the values of the nodes in the tree
+sidebar:
+  label: 'Kth Smallest Element in a BST'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Tree</Badge>
+
+::::warning
+If the BST is modified often (i.e., we can do insert and delete operations) and you need to find the kth smallest frequently, how would you optimize?
+::::
+
+### Example 1:
+- Input: `root = [3,1,4,null,2], k = 1`
+- Output: `1`
+
+### Example 2:
+- Input: `root = [5,3,6,2,4,null,null,1], k = 3`
+- Output: `3`
+
+### Constraints:
+
+- The number of nodes in the tree is n.
+- `1 <= k <= n <= 10^4`
+- `0 <= Node.val <= 10^4`
+
+## Approach
+
+```mermaid
+flowchart TD
+  S(["kthSmallest(root, k)"]) --> I["n = 0, stack = [], cur = root"]
+  I --> W{"cur or stack?"}
+  W -- no --> E(["fall off the loop — unreachable, k <= n"])
+  W -- yes --> L{"cur?"}
+  L -- yes --> P["stack.append(cur), cur = cur.left — push the whole left spine"]
+  P --> L
+  L -- no --> O["cur = stack.pop(), n += 1 — nodes pop in ascending order"]
+  O --> K{"n == k?"}
+  K -- yes --> R(["return cur.val"])
+  K -- no --> N["cur = cur.right"]
+  N --> W
+```
+
+## Solution
+
+```py
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
+        n = 0
+        stack = []
+        cur = root
+
+        while cur or stack:
+            while cur:
+                stack.append(cur)
+                cur = cur.left
+
+            cur = stack.pop()
+            n += 1
+            if n == k:
+                return cur.val
+            cur = cur.right
+```
+
+## Explanation
+
+<YouTube id="5LUXSvjmGCw" title="Kth Smallest Element in a BST - Leetcode 230 - Python" />
@@ -0,0 +1,76 @@
+---
+title: '235. Lowest Common Ancestor of a Binary Search Tree'
+description: Given a binary search tree (BST), find the lowest common ancestor (LCA) node of two given nodes in the BST
+sidebar:
+  label: 'Lowest Common Ancestor of a Binary Search Tree'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Tree</Badge>
+
+### Example 1:
+- Input: `root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 8`
+- Output: `6`
+- Explanation: The LCA of nodes `2` and `8` is `6`.
+
+### Example 2:
+- Input: `root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 4`
+- Output: `2`
+- Explanation: The LCA of nodes `2` and `4` is `2`, since a node can be a descendant of itself according to the LCA definition.
+
+### Example 3:
+- Input: `root = [2,1], p = 2, q = 1`
+- Output: `2`
+
+### Constraints:
+
+- The number of nodes in the tree is in the range [2, 10^5].
+- `-10^9 <= Node.val <= 10^9`
+- All Node.val are unique.
+- `p != q`
+- p and q will exist in the BST.
+
+## Approach
+
+```mermaid
+flowchart TD
+  S(["lowestCommonAncestor(root, p, q)"]) --> I["cur = root"]
+  I --> W{"cur?"}
+  W -- no --> E(["fall off the loop — unreachable, p and q exist in the BST"])
+  W -- yes --> A{"p.val < cur.val and q.val < cur.val?"}
+  A -- yes --> L["cur = cur.left — both sit in the left subtree"]
+  L --> W
+  A -- no --> B{"p.val > cur.val and q.val > cur.val?"}
+  B -- yes --> R["cur = cur.right — both sit in the right subtree"]
+  R --> W
+  B -- no --> C(["return cur — the paths to p and q split here"])
+```
+
+## Solution
+
+```py
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+
+class Solution:
+    def lowestCommonAncestor(
+        self, root: "TreeNode", p: "TreeNode", q: "TreeNode"
+    ) -> "TreeNode":
+        cur = root
+        while cur:
+            if p.val < cur.val and q.val < cur.val:
+                cur = cur.left
+            elif p.val > cur.val and q.val > cur.val:
+                cur = cur.right
+            else:
+                return cur
+```
+
+## Explanation
+
+<YouTube id="gs2LMfuOR9k" title="Lowest Common Ancestor of a Binary Search Tree - Leetcode 235 - Python" />
@@ -0,0 +1,67 @@
+---
+title: '98. Validate Binary Search Tree'
+description: Given the root of a binary tree, determine if it is a valid binary search tree (BST)
+sidebar:
+  label: 'Validate Binary Search Tree'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Tree</Badge>
+
+### Example 1:
+- Input: `root = [2,1,3]`
+- Output: `true`
+
+### Example 2:
+- Input: `root = [5,1,4,null,null,3,6]`
+- Output: `false`
+- Explanation: The `root` node's value is `5` but its right child's value is `4`.
+
+### Constraints:
+
+- The number of nodes in the tree is in the range [1, 10^4].
+- `-2^31 <= Node.val <= 2^31 - 1`
+
+## Approach
+
+```mermaid
+flowchart TD
+  S(["isValidBST(root)"]) --> C["valid(root, float('-inf'), float('inf')) — the root may hold any value"]
+  C --> E(["return that result"])
+  subgraph valid["valid(node, low, high)"]
+    V1{"not node?"}
+    V1 -- yes --> V2(["return True"])
+    V1 -- no --> V3{"not (low < node.val < high)?"}
+    V3 -- yes --> V4(["return False — node breaks a bound set by an ancestor"])
+    V3 -- no --> V5["valid(node.left, low, node.val) and valid(node.right, node.val, high) — each child tightens one bound"]
+    V5 --> V6(["return that result"])
+  end
+  C --> V1
+```
+
+## Solution
+
+```py
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        def valid(node, low, high):
+            if not node:
+                return True
+
+            if not (low < node.val < high):
+                return False
+
+            return valid(node.left, low, node.val) and valid(node.right, node.val, high)
+
+        return valid(root, float("-inf"), float("inf"))
+```
+
+## Explanation
+
+<YouTube id="s6ATEkipzow" title="Validate Binary Search Tree - Depth First Search - Leetcode 98" />
@@ -0,0 +1,3 @@
+import { defineMeta } from "blume";
+
+export default defineMeta({ title: "Binary Search Tree", order: 13 });
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Medium'
 ---

-<Badge variant="accent">Binary Search</Badge>
+<Badge variant="accent">Array</Badge>

 ::::warning
 Notice that rotating an array [a[0], a[1], a[2], ..., a[n-1]] 1 time results in the array [a[n-1], a[0], a[1], a[2], ..., a[n-2]].
@@ -35,6 +35,21 @@ Notice that rotating an array [a[0], a[1], a[2], ..., a[n-1]] 1 time results in
 - All the integers of `nums` are unique.
 - `nums` is sorted and rotated between 1 and `n` times.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["findMin(nums)"]) --> I["l = 0, r = n-1, lowest_index = -1"]
+  I --> W{"l <= r?"}
+  W -- no --> E(["return nums[lowest_index]"])
+  W -- yes --> M["m = (l + r) // 2"]
+  M --> C{"nums[m] <= nums[-1]?"}
+  C -- yes --> A["in the right sorted run: record lowest_index = m, r = m - 1"]
+  A --> W
+  C -- no --> B["still in the left sorted run: l = m + 1"]
+  B --> W
+```
+
 ## Solution

 ```py
@@ -53,3 +68,7 @@ class Solution:

        return nums[lowest_index]
 ```
+
+## Explanation
+
+<YouTube id="nIVW4P8b1VA" title="Find Minimum in Rotated Sorted Array - Binary Search - Leetcode 153 - Python" />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Medium'
 ---

-<Badge variant="accent">Binary Search</Badge>
+<Badge variant="accent">Array</Badge>

 ::::warning
 You must write an algorithm with O(log n) runtime complexity.
@@ -32,6 +32,29 @@ You must write an algorithm with O(log n) runtime complexity.
 - `nums` is an ascending array that is possibly rotated.
 - `-10^4 <= target <= 10^4`

+## Approach
+
+```mermaid
+flowchart TD
+  S(["search(nums, target)"]) --> I["l = 0, r = n - 1"]
+  I --> W{"l <= r?"}
+  W -- no --> E(["return -1"])
+  W -- yes --> M["mid = floor((l + r) / 2)"]
+  M --> F{"nums[mid] == target?"}
+  F -- yes --> R(["return mid"])
+  F -- no --> H{"nums[l] <= nums[mid]? — left half sorted"}
+  H -- yes --> A{"target outside [nums[l], nums[mid]]?"}
+  A -- yes --> A1["l = mid + 1 — search the right half"]
+  A -- no --> A2["r = mid - 1"]
+  H -- no --> B{"target outside [nums[mid], nums[r]]?"}
+  B -- yes --> B1["r = mid - 1 — search the left half"]
+  B -- no --> B2["l = mid + 1"]
+  A1 --> W
+  A2 --> W
+  B1 --> W
+  B2 --> W
+```
+
 ## Solution

 ```js
@@ -69,3 +92,7 @@ var search = function(nums, target) {
  return -1;
 };
 ```
+
+## Explanation
+
+<YouTube id="U8XENwh8Oy8" title="Search in rotated sorted array - Leetcode 33 - Python" />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Easy'
 ---

-<Badge variant="accent">Binary Search</Badge>
+<Badge variant="accent">Array</Badge>

 ::::warning
 You must write an algorithm with O(log n) runtime complexity.
@@ -31,6 +31,22 @@ You must write an algorithm with O(log n) runtime complexity.
 - `nums` contains distinct values sorted in ascending order.
 - `-10^4 <= target <= 10^4`

+## Approach
+
+```mermaid
+flowchart TD
+  S(["searchInsert(nums, target)"]) --> I["l = 0, r = n - 1"]
+  I --> W{"l <= r?"}
+  W -- yes --> M["m = (l + r) // 2"]
+  M --> Q{"nums[m] vs target"}
+  Q -- equal --> R(["return m"])
+  Q -- "nums[m] < target" --> L["l = m + 1"]
+  L --> W
+  Q -- "nums[m] > target" --> H["r = m - 1"]
+  H --> W
+  W -- no --> E(["return (l + r) // 2 + 1 — the insert slot"])
+```
+
 ## Solution

 ```py
@@ -49,3 +65,7 @@ class Solution:

        return (l + r) // 2 + 1
 ```
+
+## Explanation
+
+<YouTube id="K-RYzDZkzCI" title="Search Insert Position - Binary Search - Leetcode 35 - Python" />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Easy'
 ---

-<Badge variant="accent">Binary Search</Badge>
+<Badge variant="accent">Array</Badge>

 ::::warning
 You must write an algorithm with O(log n) runtime complexity.
@@ -29,6 +29,22 @@ You must write an algorithm with O(log n) runtime complexity.
 - All the integers in `nums` are unique.
 - `nums` is sorted in ascending order.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["search(nums, target)"]) --> I["left = 0, right = n - 1"]
+  I --> W{"left <= right?"}
+  W -- no --> E(["return -1"])
+  W -- yes --> M["mid = left + floor((right - left) / 2)"]
+  M --> Q{"nums[mid] vs target"}
+  Q -- equal --> R(["return mid"])
+  Q -- "nums[mid] < target" --> L["left = mid + 1"]
+  L --> W
+  Q -- "nums[mid] > target" --> H["right = mid - 1"]
+  H --> W
+```
+
 ## Solution

 ```js
@@ -54,3 +70,7 @@ var search = function(nums, target) {
    return -1;
 };
 ```
+
+## Explanation
+
+<YouTube id="s4DPM8ct1pI" title="Binary Search - Leetcode 704 - Python" />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Medium'
 ---

-<Badge variant="accent">Binary Search</Badge>
+<Badge variant="accent">Array</Badge>

 ::::warning
 You must write a solution in O(log(m * n)) time complexity.
@@ -27,6 +27,32 @@ You must write a solution in O(log(m * n)) time complexity.
 - `1 <= m, n <= 100`
 - `-10^4 <= matrix[i][j], target <= 10^4`

+## Approach
+
+```mermaid
+flowchart TD
+  S(["searchMatrix(matrix, target)"]) --> I["top = 0, bot = ROWS - 1"]
+  I --> W1{"top <= bot?"}
+  W1 -- yes --> R1["row = (top + bot) // 2"]
+  R1 --> C1{"target vs that row's range"}
+  C1 -- "above matrix[row][-1]" --> U["top = row + 1"]
+  C1 -- "below matrix[row][0]" --> V["bot = row - 1"]
+  C1 -- inside --> B(["break — row found"])
+  U --> W1
+  V --> W1
+  W1 -- no --> F(["return False — no row can hold it"])
+  B --> P["row = (top + bot) // 2; l = 0, r = COLS - 1"]
+  P --> W2{"l <= r?"}
+  W2 -- no --> G(["return False"])
+  W2 -- yes --> M["m = (l + r) // 2"]
+  M --> C2{"target vs matrix[row][m]"}
+  C2 -- greater --> X["l = m + 1"]
+  C2 -- smaller --> Y["r = m - 1"]
+  C2 -- equal --> T(["return True"])
+  X --> W2
+  Y --> W2
+```
+
 ## Solution

 ```py
@@ -59,3 +85,7 @@ class Solution:
                return True
        return False
 ```
+
+## Explanation
+
+<YouTube id="Ber2pi2C0j0" title="Search a 2D Matrix - Leetcode 74 - Python" />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Medium'
 ---

-<Badge variant="accent">Binary Search</Badge>
+<Badge variant="accent">Array</Badge>

 ### Example 1:
 - Input: `piles = [3,6,7,11], h = 8`
@@ -26,6 +26,22 @@ sidebar:
 - `piles.length <= h <= 10^9`
 - `1 <= piles[i] <= 10^9`

+## Approach
+
+```mermaid
+flowchart TD
+  S(["minEatingSpeed(piles, h)"]) --> I["l = 1, r = max(piles)"]
+  I --> W{"l < r?"}
+  W -- no --> E(["return l — the smallest workable speed"])
+  W -- yes --> M["mid = floor((l + r) / 2)"]
+  M --> K["kWorks(mid): hours = sum of ceil(p / mid)"]
+  K --> Q{"hours <= h?"}
+  Q -- yes --> A["r = mid — keep mid, try slower"]
+  Q -- no --> B["l = mid + 1 — too slow, speed up"]
+  A --> W
+  B --> W
+```
+
 ## Solution

 ```js
@@ -60,3 +76,7 @@ var minEatingSpeed = function(piles, h) {
  return l;
 };
 ```
+
+## Explanation
+
+<YouTube id="U2SozAs9RzA" title="Koko Eating Bananas - Binary Search - Leetcode 875 - Python" />
@@ -0,0 +1,3 @@
+import { defineMeta } from "blume";
+
+export default defineMeta({ title: "Binary Search", order: 5 });
@@ -0,0 +1,67 @@
+---
+title: '127. Word Ladder'
+description: 'A transformation sequence from word beginWord to word endWord using a dictionary wordList is a sequence of words beginWord -> s1 -> s2 -> ... -> sk such that:'
+sidebar:
+  label: 'Word Ladder'
+  badge: 'Hard'
+---
+
+<Badge variant="accent">Hash Table</Badge>
+
+### Example 1:
+- Input: `beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log","cog"]`
+- Output: `5`
+- Explanation: One shortest transformation sequence is "hit" -> "hot" -> "dot" -> "dog" -> cog", which is `5` words long.
+
+### Example 2:
+- Input: `beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log"]`
+- Output: `0`
+- Explanation: The `endWord` "cog" is not in `wordList`, therefore there is no valid transformation sequence.
+
+### Constraints:
+
+- `1 <= beginWord.length <= 10`
+- `endWord.length == beginWord.length`
+- `1 <= wordList.length <= 5000`
+- `wordList[i].length == beginWord.length`
+- `beginWord`, `endWord`, and `wordList[i]` consist of lowercase English letters.
+- `beginWord != endWord`
+- All the words in `wordList` are unique.
+
+## Solution
+
+```py
+import collections
+
+
+class Solution:
+    def ladderLength(self, beginWord: str, endWord: str, wordList: list[str]) -> int:
+        if endWord not in wordList:
+            return 0
+
+        adj = collections.defaultdict(list)
+        wordList.append(beginWord)
+
+        for word in wordList:
+            for j in range(len(word)):
+                pattern = word[:j] + "*" + word[j + 1 :]
+                adj[pattern].append(word)
+
+        visit = set([beginWord])
+        q = collections.deque([beginWord])
+        res = 1
+
+        while q:
+            for i in range(len(q)):
+                word = q.popleft()
+                if word == endWord:
+                    return res
+                for j in range(len(word)):
+                    pattern = word[:j] + "*" + word[j + 1 :]
+                    for adjWord in adj[pattern]:
+                        if adjWord not in visit:
+                            visit.add(adjWord)
+                            q.append(adjWord)
+            res += 1
+        return 0
+```
@@ -0,0 +1,72 @@
+---
+title: '994. Rotting Oranges'
+description: 'You are given an m x n grid where each cell can have one of three values:'
+sidebar:
+  label: 'Rotting Oranges'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Array</Badge>
+
+### Example 1:
+- Input: `grid = [[2,1,1],[1,1,0],[0,1,1]]`
+- Output: `4`
+
+### Example 2:
+- Input: `grid = [[2,1,1],[0,1,1],[1,0,1]]`
+- Output: `-1`
+- Explanation: The orange in the bottom left corner (row `2`, column `0`) is never rotten, because rotting only happens 4-directionally.
+
+### Example 3:
+- Input: `grid = [[0,2]]`
+- Output: `0`
+- Explanation: Since there are already no fresh oranges at minute `0`, the answer is just `0`.
+
+### Constraints:
+
+- `m == grid.length`
+- `n == grid[i].length`
+- `1 <= m, n <= 10`
+- grid[i][j] is 0, 1, or 2.
+
+## Solution
+
+```py
+from collections import deque
+
+
+class Solution:
+    def orangesRotting(self, grid: list[list[int]]) -> int:
+        q = deque()
+        time, fresh = 0, 0
+
+        rows, cols = len(grid), len(grid[0])
+        for r in range(rows):
+            for c in range(cols):
+                if grid[r][c] == 1:
+                    fresh += 1
+                if grid[r][c] == 2:
+                    q.append([r, c])
+
+        directions = [[0, 1], [0, -1], [1, 0], [-1, 0]]
+
+        while q and fresh > 0:
+            for i in range(len(q)):
+                r, c = q.popleft()
+                for dr, dc in directions:
+                    row, col = dr + r, dc + c
+                    # check if in bounds
+                    if (
+                        row < 0
+                        or col < 0
+                        or row == len(grid)
+                        or col == len(grid[0])
+                        or grid[row][col] != 1
+                    ):
+                        continue
+                    grid[row][col] = 2
+                    q.append([row, col])
+                    fresh -= 1
+            time += 1
+        return time if fresh == 0 else -1
+```
@@ -0,0 +1,3 @@
+import { defineMeta } from "blume";
+
+export default defineMeta({ title: "Graph BFS", order: 17 });
@@ -0,0 +1,64 @@
+---
+title: '133. Clone Graph'
+description: Given a reference of a node in a connected undirected graph
+sidebar:
+  label: 'Clone Graph'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Hash Table</Badge>
+
+### Example 1:
+- Input: `adjList = [[2,4],[1,3],[2,4],[1,3]]`
+- Output: `[[2,4],[1,3],[2,4],[1,3]]`
+- Explanation: There are `4` nodes in the graph. 1st `node` (val = 1)'s neighbors are 2nd `node` (val = `2`) and 4th `node` (val = `4`). 2nd `node` (val = 2)'s neighbors are 1st `node` (val = `1`) and 3rd `node` (val = `3`). 3rd `node` (val = 3)'s neighbors are 2nd `node` (val = `2`) and 4th `node` (val = `4`). 4th `node` (val = 4)'s neighbors are 1st `node` (val = `1`) and 3rd `node` (val = `3`).
+
+### Example 2:
+- Input: `adjList = [[]]`
+- Output: `[[]]`
+- Explanation: Note that the input contains one empty list. The graph consists of only one `node` with val = `1` and it does not have any neighbors.
+
+### Example 3:
+- Input: `adjList = []`
+- Output: `[]`
+- Explanation: This an empty graph, it does not have any nodes.
+
+### Constraints:
+
+- The number of nodes in the graph is in the range [0, 100].
+- `1 <= Node.val <= 100`
+- Node.val is unique for each node.
+- There are no repeated edges and no self-loops in the graph.
+- The Graph is connected and all nodes can be visited starting from the given node.
+
+## Solution
+
+```py
+"""
+# Definition for a Node.
+class Node:
+    def __init__(self, val = 0, neighbors = None):
+        self.val = val
+        self.neighbors = neighbors if neighbors is not None else []
+"""
+
+from typing import Optional
+
+
+class Solution:
+    def cloneGraph(self, node: Optional["Node"]) -> Optional["Node"]:
+        oldToNew = {}
+
+        def dfs(node):
+            if node in oldToNew:
+                return oldToNew[node]
+
+            copy = Node(node.val)
+            oldToNew[node] = copy
+
+            for n in node.neighbors:
+                copy.neighbors.append(dfs(n))
+            return copy
+
+        return dfs(node) if node else None
+```
@@ -0,0 +1,51 @@
+---
+title: '200. Number of Islands'
+description: Given an m x n 2D binary grid grid which represents a map of '1's (land) and '0's (water), return the number of islands
+sidebar:
+  label: 'Number of Islands'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Array</Badge>
+
+### Example 1:
+- Input: `grid = [ ["1","1","1","1","0"], ["1","1","0","1","0"], ["1","1","0","0","0"], ["0","0","0","0","0"] ]`
+- Output: `1`
+
+### Example 2:
+- Input: `grid = [ ["1","1","0","0","0"], ["1","1","0","0","0"], ["0","0","1","0","0"], ["0","0","0","1","1"] ]`
+- Output: `3`
+
+### Constraints:
+
+- `m == grid.length`
+- `n == grid[i].length`
+- `1 <= m, n <= 300`
+- grid[i][j] is '0' or '1'.
+
+## Solution
+
+```py
+class Solution:
+    def numIslands(self, grid: List[List[str]]) -> int:
+        rows, cols = len(grid), len(grid[0])
+
+        def dfs(r, c):
+            if r < 0 or r >= rows or c < 0 or c >= cols or grid[r][c] != "1":
+                return
+            else:
+                grid[r][c] = "0"
+                dfs(r, c + 1)  # right
+                dfs(r + 1, c)  # bottom
+                dfs(r, c - 1)  # left
+                dfs(r - 1, c)  # top
+
+        num_islands = 0
+        for r in range(rows):
+            for c in range(cols):
+                if grid[r][c] == "1":
+                    num_islands += 1
+                    dfs(r, c)
+
+        return num_islands
+```
@@ -0,0 +1,67 @@
+---
+title: '417. Pacific Atlantic Water Flow'
+description: There is an m x n rectangular island that borders both the Pacific Ocean and Atlantic Ocean. The Pacific Ocean touches the island's left and top edges, and the Atlantic Ocean touches the island's right and bottom edges
+sidebar:
+  label: 'Pacific Atlantic Water Flow'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Array</Badge>
+
+### Example 1:
+- Input: `heights = [[1,2,2,3,5],[3,2,3,4,4],[2,4,5,3,1],[6,7,1,4,5],[5,1,1,2,4]]`
+- Output: `[[0,4],[1,3],[1,4],[2,2],[3,0],[3,1],[4,0]]`
+- Explanation: The following cells can flow to the Pacific and Atlantic oceans, as shown below: `[0,4]`: `[0,4]` -> Pacific Ocean `[0,4]` -> Atlantic Ocean `[1,3]`: `[1,3]` -> `[0,3]` -> Pacific Ocean `[1,3]` -> `[1,4]` -> Atlantic Ocean `[1,4]`: `[1,4]` -> `[1,3]` -> `[0,3]` -> Pacific Ocean `[1,4]` -> Atlantic Ocean `[2,2]`: `[2,2]` -> `[1,2]` -> `[0,2]` -> Pacific Ocean `[2,2]` -> `[2,3]` -> `[2,4]` -> Atlantic Ocean `[3,0]`: `[3,0]` -> Pacific Ocean `[3,0]` -> `[4,0]` -> Atlantic Ocean `[3,1]`: `[3,1]` -> `[3,0]` -> Pacific Ocean `[3,1]` -> `[4,1]` -> Atlantic Ocean `[4,0]`: `[4,0]` -> Pacific Ocean `[4,0]` -> Atlantic Ocean Note that there are other possible paths for these cells to flow to the Pacific and Atlantic oceans.
+
+### Example 2:
+- Input: `heights = [[1]]`
+- Output: `[[0,0]]`
+- Explanation: The water can flow from the only cell to the Pacific and Atlantic oceans.
+
+### Constraints:
+
+- `m == heights.length`
+- `n == heights[r].length`
+- `1 <= m, n <= 200`
+- `0 <= heights[r][c] <= 10^5`
+
+## Solution
+
+```py
+class Solution:
+    def pacificAtlantic(self, heights: list[list[int]]) -> list[list[int]]:
+        rows, cols = len(heights), len(heights[0])
+        pac, atl = set(), set()
+
+        def dfs(r, c, seen, prevHeight):
+            if (
+                (r, c) in seen
+                or r < 0
+                or c < 0
+                or r == rows
+                or c == cols
+                or heights[r][c] < prevHeight
+            ):
+                return
+            seen.add((r, c))
+            dfs(r + 1, c, seen, heights[r][c])
+            dfs(r - 1, c, seen, heights[r][c])
+            dfs(r, c + 1, seen, heights[r][c])
+            dfs(r, c - 1, seen, heights[r][c])
+
+        for c in range(cols):
+            dfs(0, c, pac, heights[0][c])
+            dfs(rows - 1, c, atl, heights[rows - 1][c])
+
+        for r in range(rows):
+            dfs(r, 0, pac, heights[r][0])
+            dfs(r, cols - 1, atl, heights[r][cols - 1])
+
+        res = []
+        for r in range(rows):
+            for c in range(cols):
+                if (r, c) in pac and (r, c) in atl:
+                    res.append([r, c])
+
+        return res
+```
@@ -0,0 +1,3 @@
+import { defineMeta } from "blume";
+
+export default defineMeta({ title: "Graph DFS", order: 16 });
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Easy'
 ---

-<Badge variant="accent">Hash Table</Badge>
+<Badge variant="accent">Array</Badge>

 ::::warning
 Can you come up with an algorithm that is less than O(n^2) time complexity?
@@ -32,6 +32,20 @@ Can you come up with an algorithm that is less than O(n^2) time complexity?
 - `-10^9 <= target <= 10^9`
 - Only one valid answer exists.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["twoSum(nums, target)"]) --> I["seen = {} — value to index"]
+  I --> L{"more (i, num) in nums?"}
+  L -- no --> E(["return []"])
+  L -- yes --> C["complement = target - num"]
+  C --> H{"complement in seen?"}
+  H -- yes --> R(["return [seen[complement], i]"])
+  H -- no --> W["seen[num] = i"]
+  W --> L
+```
+
 ## Solution

 ```py
@@ -45,3 +59,7 @@ class Solution:
            seen[num] = i
        return []
 ```
+
+## Explanation
+
+<YouTube id="KLlXCFG5TnA" title="Two Sum - Leetcode 1 - HashMap - Python" />
@@ -22,6 +22,19 @@ sidebar:
 - `1 <= sentence.length <= 1000`
 - `sentence` consists of lowercase English letters.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["checkIfPangram(sentence)"]) --> I["freq = {}"]
+  I --> L{"more char c?"}
+  L -- yes --> C["freq[c] = (freq[c] or 0) + 1"]
+  C --> L
+  L -- no --> Q{"distinct keys in freq == 26?"}
+  Q -- yes --> R(["return true"])
+  Q -- no --> F(["return false"])
+```
+
 ## Solution

 ```js
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Easy'
 ---

-<Badge variant="accent">Hash Table</Badge>
+<Badge variant="accent">Array</Badge>

 ### Example 1:
 - Input: `nums = [1,2,3,1]`
@@ -27,9 +27,29 @@ sidebar:
 - `1 <= nums.length <= 10^5`
 - `-10^9 <= nums[i] <= 10^9`

+## Approach
+
+```mermaid
+flowchart TD
+  S(["containsDuplicate(nums)"]) --> I["freq = {}"]
+  I --> L{"more n in nums?"}
+  L -- no --> E(["return false"])
+  L -- yes --> C["freq[n] = (freq[n] or 0) + 1"]
+  C --> Q{"freq[n] >= 2?"}
+  Q -- yes --> R(["return true"])
+  Q -- no --> L
+```
+
 ## Solution

-```js
+<CodeGroup>
+
+```py Python
+class Solution:
+    def containsDuplicate(self, nums: List[int]) -> bool:
+```
+
+```js JavaScript
 /**
 * @param {number[]} nums
 * @return {boolean}
@@ -45,3 +65,9 @@ var containsDuplicate = function(nums) {
  return false;
 };
 ```
+
+</CodeGroup>
+
+## Explanation
+
+<YouTube id="3OamzN90kPg" title="Contains Duplicate - Leetcode 217 - Python" />
@@ -25,6 +25,19 @@ What if the inputs contain Unicode characters? How would you adapt your solution
 - `1 <= s.length, t.length <= 5 * 10^4`
 - `s` and `t` consist of lowercase English letters.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["isAnagram(s, t)"]) --> I["freq: count every char of s"]
+  I --> L{"more char c in t?"}
+  L -- no --> E(["return true"])
+  L -- yes --> Q{"freq[c] missing or 0?"}
+  Q -- yes --> R(["return false — t has a char s cannot cover"])
+  Q -- no --> D["freq[c] -= 1"]
+  D --> L
+```
+
 ## Solution

 ```js
@@ -46,3 +59,7 @@ var isAnagram = function(s, t) {
  return true;
 };
 ```
+
+## Explanation
+
+<YouTube id="9UtInBqnCgA" title="Valid Anagram - Leetcode 242 - Python" />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Medium'
 ---

-<Badge variant="accent">Hash Table</Badge>
+<Badge variant="accent">Array</Badge>

 ::::warning
 Your algorithm's time complexity must be better than O(n log n), where n is the array's size.
@@ -31,6 +31,16 @@ Your algorithm's time complexity must be better than O(n log n), where n is the
 - `k` is in the range `[1, the number of unique elements in the array]`.
 - It is guaranteed that the answer is unique.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["topKFrequent(nums, k)"]) --> F["freq: count every value in nums"]
+  F --> K["keys of freq, cast to Number"]
+  K --> O["sort by freq[b] - freq[a] — most frequent first"]
+  O --> R(["return slice(0, k)"])
+```
+
 ## Solution

 ```js
@@ -49,3 +59,7 @@ var topKFrequent = function (nums, k) {
    .slice(0, k);
 };
 ```
+
+## Explanation
+
+<YouTube id="YPTqKIgVk-k" title="Top K Frequent Elements - Bucket Sort - Leetcode 347 - Python" />
@@ -26,6 +26,18 @@ sidebar:
 - `1 <= s.length <= 10^5`
 - `s` consists of only lowercase English letters.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["firstUniqChar(s)"]) --> F["Pass 1 — freq: count every char of s"]
+  F --> L{"Pass 2 — more i in 0..len-1?"}
+  L -- no --> E(["return -1"])
+  L -- yes --> Q{"freq[s[i]] == 1?"}
+  Q -- yes --> R(["return i — leftmost wins because the scan is in order"])
+  Q -- no --> L
+```
+
 ## Solution

 ```js
@@ -45,3 +57,7 @@ var firstUniqChar = function (s) {
  return -1;
 };
 ```
+
+## Explanation
+
+<YouTube id="rBENYgWy3xU" title="First Unique Character in a String - Leetcode 387 - Python" />
@@ -28,6 +28,18 @@ sidebar:
 - `1 <= s.length <= 5 * 10^5`
 - `s` consists of uppercase and lowercase English letters and digits.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["frequencySort(s)"]) --> F["freq: count every char of s"]
+  F --> C["split s and sort with a two-key comparator"]
+  C --> K1["primary: freq[b] - freq[a] — most frequent first"]
+  C --> K2["tie-break: a.localeCompare(b) — alphabetical"]
+  K1 --> R(["join and return"])
+  K2 --> R
+```
+
 ## Solution

 ```js
@@ -47,3 +59,7 @@ var frequencySort = function (s) {
    .join("");
 };
 ```
+
+## Explanation
+
+<YouTube id="OXdXc9HTrIg" title="Sort Characters By Frequency - Leetcode 451 - Python" />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Medium'
 ---

-<Badge variant="accent">Hash Table</Badge>
+<Badge variant="accent">Array</Badge>

 ### Example 1:
 - Input: `strs = ["eat","tea","tan","ate","nat","bat"]`
@@ -27,6 +27,21 @@ sidebar:
 - `0 <= strs[i].length <= 100`
 - `strs[i]` consists of lowercase English letters.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["groupAnagrams(strs)"]) --> K["sorted[i] = letters of strs[i], sorted — the anagram key"]
+  K --> I["anagrams = {}"]
+  I --> L{"more i in 0..n-1?"}
+  L -- no --> E(["return Object.values(anagrams)"])
+  L -- yes --> Q{"anagrams has sorted[i]?"}
+  Q -- no --> N["anagrams[sorted[i]] = [strs[i]]"]
+  Q -- yes --> P["anagrams[sorted[i]].push(strs[i])"]
+  N --> L
+  P --> L
+```
+
 ## Solution

 ```js
@@ -49,3 +64,7 @@ var groupAnagrams = function(strs) {
  return Object.values(anagrams);
 };
 ```
+
+## Explanation
+
+<YouTube id="vzdNOK2oB2E" title="Group Anagrams - Categorize Strings by Count - Leetcode 49" />
@@ -0,0 +1,3 @@
+import { defineMeta } from "blume";
+
+export default defineMeta({ title: "Hash Table", order: 1 });
@@ -0,0 +1,59 @@
+---
+title: '215. Kth Largest Element in an Array'
+description: Given an integer array nums and an integer k, return the k^th largest element in the array
+sidebar:
+  label: 'Kth Largest Element in an Array'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Array</Badge>
+
+### Example 1:
+- Input: `nums = [3,2,1,5,6,4], k = 2`
+- Output: `5`
+
+### Example 2:
+- Input: `nums = [3,2,3,1,2,4,5,5,6], k = 4`
+- Output: `4`
+
+### Constraints:
+
+- `1 <= k <= nums.length <= 10^5`
+- `-10^4 <= nums[i] <= 10^4`
+
+## Approach
+
+```mermaid
+flowchart TD
+  S(["findKthLargest(nums, k)"]) --> F{"more i in range(len(nums))?"}
+  F -- yes --> N["nums[i] = -nums[i] — negate so the min-heap acts as a max-heap"]
+  N --> F
+  F -- no --> H["heapq.heapify(nums)"]
+  H --> W{"more _ in range(k - 1)?"}
+  W -- yes --> P["heapq.heappop(nums) — discard the k-1 largest"]
+  P --> W
+  W -- no --> E(["return -heapq.heappop(nums)"])
+```
+
+## Solution
+
+```py
+import heapq
+
+
+class Solution:
+    def findKthLargest(self, nums: List[int], k: int) -> int:
+        for i in range(len(nums)):
+            nums[i] = -nums[i]
+
+        heapq.heapify(nums)
+
+        for _ in range(k - 1):
+            heapq.heappop(nums)
+
+        return -heapq.heappop(nums)
+```
+
+## Explanation
+
+<YouTube id="XEmy13g1Qxc" title="Kth Largest Element in an Array - Quick Select - Leetcode 215 - Python" />
@@ -0,0 +1,94 @@
+---
+title: '23. Merge k Sorted Lists'
+description: You are given an array of k linked-lists lists, each linked-list is sorted in ascending order
+sidebar:
+  label: 'Merge k Sorted Lists'
+  badge: 'Hard'
+---
+
+<Badge variant="accent">Linked List</Badge>
+
+### Example 1:
+- Input: `lists = [[1,4,5],[1,3,4],[2,6]]`
+- Output: `[1,1,2,3,4,4,5,6]`
+- Explanation: The linked-lists are: [ 1->4->5, 1->3->4, 2->6 ] merging them into one sorted linked list: 1->1->2->3->4->4->5->6
+
+### Example 2:
+- Input: `lists = []`
+- Output: `[]`
+
+### Example 3:
+- Input: `lists = [[]]`
+- Output: `[]`
+
+### Constraints:
+
+- `k == lists.length`
+- `0 <= k <= 10^4`
+- `0 <= lists[i].length <= 500`
+- `-10^4 <= lists[i][j] <= 10^4`
+- `lists[i]` is sorted in ascending order.
+- The sum of lists[i].length will not exceed 10^4.
+
+## Approach
+
+```mermaid
+flowchart TD
+  S(["mergeKLists(lists)"]) --> I["heap = []"]
+  I --> F{"more i, node in enumerate(lists)?"}
+  F -- yes --> C{"node?"}
+  C -- yes --> P["heapq.heappush(heap, (node.val, i, node)) — i breaks ties so nodes are never compared"]
+  P --> F
+  C -- no --> F
+  F -- no --> D["D = ListNode(), cur = D"]
+  D --> W{"heap?"}
+  W -- no --> E(["return D.next"])
+  W -- yes --> O["val, i, node = heapq.heappop(heap) — the smallest head across all lists"]
+  O --> L["cur.next = node, cur = node, node = node.next"]
+  L --> N{"node?"}
+  N -- yes --> Q["heapq.heappush(heap, (node.val, i, node))"]
+  Q --> W
+  N -- no --> W
+```
+
+## Solution
+
+```py
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+import heapq
+
+
+class Solution:
+    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
+        heap = []
+
+        # K log K
+        for i, node in enumerate(lists):
+            if node:
+                heapq.heappush(heap, (node.val, i, node))
+
+        D = ListNode()
+        cur = D
+
+        # n log k
+        while heap:
+            val, i, node = heapq.heappop(heap)
+            cur.next = node
+            cur = node
+            node = node.next
+
+            if node:
+                heapq.heappush(heap, (node.val, i, node))
+
+        # Time: O(N log k)
+        # Space: O(n)
+        return D.next
+```
+
+## Explanation
+
+<YouTube id="q5a5OiGbT6Q" title="Merge K Sorted Lists - Leetcode 23 - Python" />
@@ -0,0 +1,69 @@
+---
+title: '973. K Closest Points to Origin'
+description: Given an array of points where points[i] = [xi, yi] represents a point on the X-Y plane and an integer k, return the k closest points to the origin (0, 0)
+sidebar:
+  label: 'K Closest Points to Origin'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Array</Badge>
+
+### Example 1:
+- Input: `points = [[1,3],[-2,2]], k = 1`
+- Output: `[[-2,2]]`
+- Explanation: The distance between `(1`, `3`) and the origin is sqrt(10). The distance between `(-2`, `2`) and the origin is sqrt(8). Since sqrt(8) < sqrt(10), `(-2`, `2`) is closer to the origin. We only want the closest `k = 1 points` from the origin, so the answer is just [[-2,2]].
+
+### Example 2:
+- Input: `points = [[3,3],[5,-1],[-2,4]], k = 2`
+- Output: `[[3,3],[-2,4]]`
+- Explanation: The answer [[-2,4],[3,3]] would also be accepted.
+
+### Constraints:
+
+- `1 <= k <= points.length <= 10^4`
+- `-10^4 <= xi, yi <= 10^4`
+
+## Approach
+
+```mermaid
+flowchart TD
+  S(["kClosest(points, k)"]) --> I["minHeap = []"]
+  I --> F{"more x, y in points?"}
+  F -- yes --> D["dist = x**2 + y**2 — squared distance orders the same as sqrt"]
+  D --> A["minHeap.append([dist, x, y])"]
+  A --> F
+  F -- no --> H["heapq.heapify(minHeap), res = [] — O(n) build, smallest dist on top"]
+  H --> W{"k > 0?"}
+  W -- yes --> P["dist, x, y = heapq.heappop(minHeap)"]
+  P --> R["res.append([x, y]), k -= 1"]
+  R --> W
+  W -- no --> E(["return res"])
+```
+
+## Solution
+
+```py
+import heapq
+
+
+class Solution:
+    def kClosest(self, points: List[List[int]], k: int) -> List[List[int]]:
+        minHeap = []
+        for x, y in points:
+            dist = (x**2) + (y**2)
+            minHeap.append([dist, x, y])
+
+        heapq.heapify(minHeap)
+        res = []
+
+        while k > 0:
+            dist, x, y = heapq.heappop(minHeap)
+            res.append([x, y])
+            k -= 1
+
+        return res
+```
+
+## Explanation
+
+<YouTube id="rI2EBUEMfTk" title="K Closest Points to Origin - Heap / Priority Queue - Leetcode 973 - Python" />
@@ -0,0 +1,3 @@
+import { defineMeta } from "blume";
+
+export default defineMeta({ title: "Heap / Priority Queue", order: 14 });
@@ -0,0 +1,115 @@
+---
+title: '146. LRU Cache'
+description: Design a data structure that follows the constraints of a Least Recently Used (LRU) cache
+sidebar:
+  label: 'LRU Cache'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Hash Table</Badge>
+
+### Example 1:
+- Input: ``
+- Output: ``
+- Explanation: Input ["LRUCache", "put", "put", "get", "put", "get", "put", "get", "get", "get"] `[[2]`, [1, 1], [2, 2], `[1]`, [3, 3], `[2]`, [4, 4], `[1]`, `[3]`, [4]] Output [null, null, null, `1`, null, `-1`, null, `-1`, `3`, 4] Explanation LRUCache lRUCache = new LRUCache(2); lRUCache.put(1, `1`); // cache is {1=1} lRUCache.put(2, `2`); // cache is {1=1, 2=2} lRUCache.get(1);    // return `1` lRUCache.put(3, `3`); // LRU `key` was `2`, evicts `key 2`, cache is {1=1, 3=3} lRUCache.get(2);    // returns `-1` (not found) lRUCache.put(4, `4`); // LRU `key` was `1`, evicts `key 1`, cache is {4=4, 3=3} lRUCache.get(1);    // return `-1` (not found) lRUCache.get(3);    // return `3` lRUCache.get(4);    // return `4`
+
+### Constraints:
+
+- `1 <= capacity <= 3000`
+- `0 <= key <= 10^4`
+- `0 <= value <= 10^5`
+- At most 2 * 10^5 calls will be made to get and put.
+
+## Approach
+
+```mermaid
+flowchart TD
+  subgraph state["state — dict for O(1) lookup, doubly linked list for O(1) reorder"]
+    H["head — least recent"] --- N["... nodes ..."] --- T["tail — most recent"]
+  end
+  subgraph get["get(key)"]
+    G1{"key in cache?"} -- no --> G2(["return -1"])
+    G1 -- yes --> G3["_remove_node then _add_node — move it beside the tail"]
+    G3 --> G4(["return node.value"])
+  end
+  subgraph put["put(key, value)"]
+    P1{"key in cache?"} -- yes --> P2["update value, _remove_node then _add_node"]
+    P1 -- no --> P3["new Node, cache[key] = node, _add_node"]
+    P3 --> P4{"len(cache) > capacity?"}
+    P4 -- yes --> P5["evict head.next — the least recent — and del cache[lru.key]"]
+    P4 -- no --> P6(["done"])
+    P2 --> P6
+    P5 --> P6
+  end
+  state --> get
+  state --> put
+```
+
+## Solution
+
+```py
+class Node:
+    def __init__(self, key=0, value=0):
+        self.key = key
+        self.value = value
+        self.prev = None
+        self.next = None
+
+
+class LRUCache:
+    def __init__(self, capacity: int):
+        self.capacity = capacity
+        self.cache = {}
+        self.head = Node()
+        self.tail = Node()
+        self.head.next = self.tail  # pyright: ignore[reportAttributeAccessIssue]
+        self.tail.prev = self.head  # pyright: ignore[reportAttributeAccessIssue]
+
+    def _remove_node(self, node):
+        prev_node = node.prev
+        next_node = node.next
+        prev_node.next = next_node
+        next_node.prev = prev_node
+
+    def _add_node(self, node):
+        # Insert just before the tail (most recent at end)
+        node.next = self.tail
+        node.prev = self.tail.prev
+        self.tail.prev.next = node  # pyright: ignore[reportAttributeAccessIssue]
+        self.tail.prev = node
+
+    def get(self, key: int) -> int:
+        if key not in self.cache:
+            return -1
+
+        node = self.cache[key]
+        self._remove_node(node)
+        self._add_node(node)
+        return node.value
+
+    def put(self, key: int, value: int) -> None:
+        if key in self.cache:
+            node = self.cache[key]
+            node.value = value
+            self._remove_node(node)
+            self._add_node(node)
+        else:
+            new_node = Node(key, value)
+            self.cache[key] = new_node
+            self._add_node(new_node)
+
+            if len(self.cache) > self.capacity:
+                lru = self.head.next  # Oldest node is next to the head
+                self._remove_node(lru)
+                del self.cache[lru.key]  # pyright: ignore[reportAttributeAccessIssue]
+
+
+# Your LRUCache object will be instantiated and called as such:
+# obj = LRUCache(capacity)
+# param_1 = obj.get(key)
+# obj.put(key,value)
+```
+
+## Explanation
+
+<YouTube id="7ABFKPK2hD4" title="LRU Cache - Twitch Interview Question - Leetcode 146" />
@@ -0,0 +1,75 @@
+---
+title: '2. Add Two Numbers'
+description: You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list
+sidebar:
+  label: 'Add Two Numbers'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Linked List</Badge>
+
+### Example 1:
+- Input: `l1 = [2,4,3], l2 = [5,6,4]`
+- Output: `[7,0,8]`
+- Explanation: `342 + 465 = 807`.
+
+### Example 2:
+- Input: `l1 = [0], l2 = [0]`
+- Output: `[0]`
+
+### Example 3:
+- Input: `l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]`
+- Output: `[8,9,9,9,0,0,0,1]`
+
+### Constraints:
+
+- The number of nodes in each linked list is in the range [1, 100].
+- `0 <= Node.val <= 9`
+- It is guaranteed that the list represents a number that does not have leading zeros.
+
+## Approach
+
+```mermaid
+flowchart TD
+  S(["addTwoNumbers(l1, l2)"]) --> I["dummy = ListNode(), carry = 0, curr = dummy — dummy.next will be the head"]
+  I --> W{"l1 or l2 or carry?"}
+  W -- no --> E(["return dummy.next"])
+  W -- yes --> A["s = (l1.val if l1 else 0) + (l2.val if l2 else 0) + carry — a missing digit counts as 0"]
+  A --> D["carry, val = divmod(s, 10)"]
+  D --> N["curr.next = ListNode(val), curr = curr.next"]
+  N --> V["l1 = l1.next if l1 else None, l2 = l2.next if l2 else None"]
+  V --> W
+```
+
+## Solution
+
+```py
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def addTwoNumbers(
+        self, l1: Optional[ListNode], l2: Optional[ListNode]
+    ) -> Optional[ListNode]:
+
+        dummy = ListNode()
+        carry = 0
+        curr = dummy
+        while l1 or l2 or carry:
+            s = (l1.val if l1 else 0) + (l2.val if l2 else 0) + carry
+            carry, val = divmod(s, 10)
+
+            curr.next = ListNode(val)
+            curr = curr.next
+
+            l1 = l1.next if l1 else None
+            l2 = l2.next if l2 else None
+
+        return dummy.next
+```
+
+## Explanation
+
+<YouTube id="wgFPrzTjm7s" title="Add Two Numbers - Leetcode 2 - Python" />
@@ -0,0 +1,3 @@
+import { defineMeta } from "blume";
+
+export default defineMeta({ title: "Hybrid Structures", order: 9 });
@@ -1,12 +1,11 @@
 ---
-title: Introduction
-description: Welcome to your new Blume docs.
+title: Overview
+description: Live SRS campaign dashboard — phases, ladder, review queue, gates, and recent attempts, fetched from the SRS Worker.
 ---

-## Getting Started
+<DependencySpine />

-![Dependency Map](../public/dependency-spine.svg)
+Live view of the 56-day campaign. Everything below is fetched client-side
+from the SRS Worker, so it is always current — no rebuild required.

-Welcome to **Blume** — markdown-first docs powered by Astro and Vite.
-
-Edit `docs/index.mdx` to get started, then run `blume dev`.
+<ProgressDashboard />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Easy'
 ---

-<Badge variant="accent">Linked List</Badge>
+<Badge variant="accent">Hash Table</Badge>

 ::::warning
 Can you solve it using O(1) (i.e. constant) memory?
@@ -33,6 +33,20 @@ Can you solve it using O(1) (i.e. constant) memory?
 - `-10^5 <= Node.val <= 10^5`
 - pos is -1 or a valid index in the linked-list.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["hasCycle(head)"]) --> I["fast = slow = head"]
+  I --> W{"fast and fast.next?"}
+  W -- no --> E(["return False — ran off the end, no cycle"])
+  W -- yes --> A["fast = fast.next.next — two steps"]
+  A --> B["slow = slow.next — one step"]
+  B --> Q{"fast is slow?"}
+  Q -- yes --> R(["return True — the gap closed, so it loops"])
+  Q -- no --> W
+```
+
 ## Solution

 ```py
@@ -54,3 +68,7 @@ class Solution:
                return True
        return False
 ```
+
+## Explanation
+
+<YouTube id="gBTe7lFR3vc" title="Linked List Cycle - Floyd's Tortoise and Hare - Leetcode 141 - Python" />
@@ -29,6 +29,19 @@ A linked list can be reversed either iteratively or recursively. Could you imple
 - The number of nodes in the list is the range [0, 5000].
 - `-5000 <= Node.val <= 5000`

+## Approach
+
+```mermaid
+flowchart TD
+  S(["reverseList(head)"]) --> I["prev = None, curr = head"]
+  I --> W{"curr?"}
+  W -- no --> E(["return prev — the old tail is the new head"])
+  W -- yes --> A["next_ = curr.next — save it before overwriting"]
+  A --> B["curr.next = prev — flip the link"]
+  B --> C["prev = curr; curr = next_ — step forward"]
+  C --> W
+```
+
 ## Solution

 ```py
@@ -48,3 +61,7 @@ class Solution:
            curr = next_
        return prev
 ```
+
+## Explanation
+
+<YouTube id="G0_I-ZF0S38" title="Reverse Linked List - Iterative AND Recursive - Leetcode 206 - Python" />
@@ -26,6 +26,23 @@ sidebar:
 - `-100 <= Node.val <= 100`
 - Both list1 and list2 are sorted in non-decreasing order.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["mergeTwoLists(list1, list2)"]) --> G1{"list1 empty?"}
+  G1 -- yes --> X1(["return list2"])
+  G1 -- no --> G2{"list2 empty?"}
+  G2 -- yes --> X2(["return list1"])
+  G2 -- no --> H["head = the smaller first node; advance that list"]
+  H --> C["current = head"]
+  C --> W{"list1 and list2 both non-empty?"}
+  W -- yes --> P["current.next = the smaller node; advance that list; current = current.next"]
+  P --> W
+  W -- no --> T["current.next = list1 or list2 — append the leftover tail"]
+  T --> E(["return head"])
+```
+
 ## Solution

 ```py
@@ -64,3 +81,7 @@ class Solution:
        current.next = list1 or list2
        return head
 ```
+
+## Explanation
+
+<YouTube id="XIdigk956u0" title="Merge Two Sorted Lists - Leetcode 21 - Python" />
@@ -0,0 +1,3 @@
+import { defineMeta } from "blume";
+
+export default defineMeta({ title: "Linked List", order: 8 });
@@ -0,0 +1,57 @@
+---
+title: '48. Rotate Image'
+description: You are given an n x n 2D matrix representing an image, rotate the image by 90 degrees (clockwise)
+sidebar:
+  label: 'Rotate Image'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Array</Badge>
+
+### Example 1:
+- Input: `matrix = [[1,2,3],[4,5,6],[7,8,9]]`
+- Output: `[[7,4,1],[8,5,2],[9,6,3]]`
+
+### Example 2:
+- Input: `matrix = [[5,1,9,11],[2,4,8,10],[13,3,6,7],[15,14,12,16]]`
+- Output: `[[15,13,2,5],[14,3,4,1],[12,6,8,9],[16,7,10,11]]`
+
+### Constraints:
+
+- `n == matrix.length == matrix[i].length`
+- `1 <= n <= 20`
+- `-1000 <= matrix[i][j] <= 1000`
+
+## Approach
+
+```mermaid
+flowchart TD
+  S(["rotate(matrix)"]) --> N["n = len(matrix)"]
+  N --> F{"more r in range(n)?"}
+  F -- yes --> G{"more c in range(r + 1, n)?"}
+  G -- yes --> T["swap matrix[r][c] and matrix[c][r] — transpose across the diagonal"]
+  T --> G
+  G -- no --> F
+  F -- no --> H{"more r in range(n)?"}
+  H -- yes --> R["matrix[r].reverse() — transpose then mirror is a clockwise turn"]
+  R --> H
+  H -- no --> E(["return None — rotated in place"])
+```
+
+## Solution
+
+```py
+class Solution:
+    def rotate(self, matrix: List[List[int]]) -> None:
+        n = len(matrix)
+        for r in range(n):
+            for c in range(r + 1, n):
+                matrix[r][c], matrix[c][r] = matrix[c][r], matrix[r][c]
+
+        for r in range(n):
+            matrix[r].reverse()
+```
+
+## Explanation
+
+<YouTube id="fMSJSS7eO1w" title="Rotate Image - Matrix - Leetcode 48" />
@@ -0,0 +1,81 @@
+---
+title: '54. Spiral Matrix'
+description: Given an m x n matrix, return all elements of the matrix in spiral order
+sidebar:
+  label: 'Spiral Matrix'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Array</Badge>
+
+### Example 1:
+- Input: `matrix = [[1,2,3],[4,5,6],[7,8,9]]`
+- Output: `[1,2,3,6,9,8,7,4,5]`
+
+### Example 2:
+- Input: `matrix = [[1,2,3,4],[5,6,7,8],[9,10,11,12]]`
+- Output: `[1,2,3,4,8,12,11,10,9,5,6,7]`
+
+### Constraints:
+
+- `m == matrix.length`
+- `n == matrix[i].length`
+- `1 <= m, n <= 10`
+- `-100 <= matrix[i][j] <= 100`
+
+## Approach
+
+```mermaid
+flowchart TD
+  S(["spiralOrder(matrix)"]) --> I["res = [], top = 0, bottom = len(matrix) - 1, left = 0, right = len(matrix[0]) - 1"]
+  I --> W{"top <= bottom and left <= right?"}
+  W -- no --> E(["return res"])
+  W -- yes --> T["append matrix[top][c] for c in left..right, then top += 1 — top row, shrink from above"]
+  T --> R["append matrix[r][right] for r in top..bottom, then right -= 1 — right column"]
+  R --> B{"top <= bottom?"}
+  B -- yes --> C["append matrix[bottom][c] for c in right down to left, then bottom -= 1 — bottom row"]
+  C --> L{"left <= right?"}
+  B -- no --> L
+  L -- yes --> D["append matrix[r][left] for r in bottom down to top, then left += 1 — left column"]
+  D --> W
+  L -- no --> W
+```
+
+## Solution
+
+```py
+class Solution:
+    def spiralOrder(self, matrix: List[List[int]]) -> List[int]:
+        res = []
+        top, bottom = 0, len(matrix) - 1
+        left, right = 0, len(matrix[0]) - 1
+
+        while top <= bottom and left <= right:
+            # 1. Top Row
+            for c in range(left, right + 1):
+                res.append(matrix[top][c])
+            top += 1
+
+            # 2. Right Column
+            for r in range(top, bottom + 1):
+                res.append(matrix[r][right])
+            right -= 1
+
+            # 3. Bottom Row
+            if top <= bottom:
+                for c in range(right, left - 1, -1):
+                    res.append(matrix[bottom][c])
+                bottom -= 1
+
+            # 4. Left Column
+            if left <= right:
+                for r in range(bottom, top - 1, -1):
+                    res.append(matrix[r][left])
+                left += 1
+
+        return res
+```
+
+## Explanation
+
+<YouTube id="BJnMZNwUk1M" title="Spiral Matrix - Microsoft Interview Question - Leetcode 54" />
@@ -0,0 +1,100 @@
+---
+title: '73. Set Matrix Zeroes'
+description: Given an m x n integer matrix matrix, if an element is 0, set its entire row and column to 0's
+sidebar:
+  label: 'Set Matrix Zeroes'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Array</Badge>
+
+::::warning
+You must do it in place.
+::::
+
+### Example 1:
+- Input: `matrix = [[1,1,1],[1,0,1],[1,1,1]]`
+- Output: `[[1,0,1],[0,0,0],[1,0,1]]`
+
+### Example 2:
+- Input: `matrix = [[0,1,2,0],[3,4,5,2],[1,3,1,5]]`
+- Output: `[[0,0,0,0],[0,4,5,0],[0,3,1,0]]`
+
+### Constraints:
+
+- `m == matrix.length`
+- `n == matrix[0].length`
+- `1 <= m, n <= 200`
+- `-2^31 <= matrix[i][j] <= 2^31 - 1`
+- A straightforward solution using O(mn) space is probably a bad idea.
+- A simple improvement uses O(m + n) space, but still not the best solution.
+- Could you devise a constant space solution?
+
+## Approach
+
+```mermaid
+flowchart TD
+  S(["setZeroes(matrix)"]) --> I["m = len(matrix), n = len(matrix[0])"]
+  I --> Z["row0_zero = any(matrix[0][c] == 0), col0_zero = any(matrix[r][0] == 0) — remember before they become markers"]
+  Z --> F{"more r in range(1, m)?"}
+  F -- yes --> G{"more c in range(1, n)?"}
+  G -- yes --> Q{"matrix[r][c] == 0?"}
+  Q -- yes --> M["matrix[r][0] = 0, matrix[0][c] = 0 — row 0 and column 0 are the marker lists"]
+  M --> G
+  Q -- no --> G
+  G -- no --> F
+  F -- no --> H{"more r in range(1, m)?"}
+  H -- yes --> J{"more c in range(1, n)?"}
+  J -- yes --> K{"matrix[r][0] == 0 or matrix[0][c] == 0?"}
+  K -- yes --> W["matrix[r][c] = 0"]
+  W --> J
+  K -- no --> J
+  J -- no --> H
+  H -- no --> A{"row0_zero?"}
+  A -- yes --> B["matrix[0][c] = 0 for c in range(n)"]
+  B --> C{"col0_zero?"}
+  A -- no --> C
+  C -- yes --> D["matrix[r][0] = 0 for r in range(m)"]
+  D --> E(["return None — zeroed in place"])
+  C -- no --> E
+```
+
+## Solution
+
+```py
+class Solution:
+    def setZeroes(self, matrix: List[List[int]]) -> None:
+        """
+        Do not return anything, modify matrix in-place instead.
+        """
+        m, n = len(matrix), len(matrix[0])
+
+        # 1. remember if row 0 or column 0 have their own zeros
+        row0_zero = any(matrix[0][c] == 0 for c in range(n))
+        col0_zero = any(matrix[r][0] == 0 for r in range(m))
+
+        # 2. use row 0 and column 0 as marker lists
+        for r in range(1, m):
+            for c in range(1, n):
+                if matrix[r][c] == 0:
+                    matrix[r][0] = 0
+                    matrix[0][c] = 0
+
+        # 3. zero the inner cells from the markers
+        for r in range(1, m):
+            for c in range(1, n):
+                if matrix[r][0] == 0 or matrix[0][c] == 0:
+                    matrix[r][c] = 0
+
+        # 4. handle row 0 and column 0 last
+        if row0_zero:
+            for c in range(n):
+                matrix[0][c] = 0
+        if col0_zero:
+            for r in range(m):
+                matrix[r][0] = 0
+```
+
+## Explanation
+
+<YouTube id="T41rL0L3Pnw" title="Set Matrix Zeroes - In-place - Leetcode 73" />
@@ -0,0 +1,3 @@
+import { defineMeta } from "blume";
+
+export default defineMeta({ title: "Matrix Index Math", order: 10 });
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Medium'
 ---

-<Badge variant="accent">Monotonic Stack</Badge>
+<Badge variant="accent">Array</Badge>

 ### Example 1:
 - Input: `temperatures = [73,74,75,71,69,72,76,73]`
@@ -25,6 +25,20 @@ sidebar:
 - `1 <= temperatures.length <= 10^5`
 - `30 <= temperatures[i] <= 100`

+## Approach
+
+```mermaid
+flowchart TD
+  S(["dailyTemperatures(temperatures)"]) --> I["ans = zeros(n), stack = [] — indices of days still waiting"]
+  I --> L{"more (i, temp)?"}
+  L -- no --> E(["return ans"])
+  L -- yes --> W{"stack non-empty and temperatures[stack[-1]] < temp?"}
+  W -- yes --> P["prev = stack.pop(); ans[prev] = i - prev — today resolves that day"]
+  P --> W
+  W -- no --> A["stack.append(i)"]
+  A --> L
+```
+
 ## Solution

 ```py
@@ -39,3 +53,7 @@ class Solution:
            stack.append(i)
        return ans
 ```
+
+## Explanation
+
+<YouTube id="cTBiBSnjO3c" title="Daily Temperatures - Monotonic Stack - Leetcode 739 - Python" />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Medium'
 ---

-<Badge variant="accent">Monotonic Stack</Badge>
+<Badge variant="accent">Array</Badge>

 ### Example 1:
 - Input: `target = 12, position = [10,8,0,5,3], speed = [2,4,1,1,3]`
@@ -32,6 +32,22 @@ sidebar:
 - All the values of `position` are unique.
 - `0 < speed[i] <= 10^6`

+## Approach
+
+```mermaid
+flowchart TD
+  S(["carFleet(target, position, speed)"]) --> O["zip and sort by position, descending — nearest the target first"]
+  O --> I["stack = [] — arrival time of each fleet's leader"]
+  I --> L{"more (pos, spd)?"}
+  L -- no --> E(["return len(stack) — one entry per fleet"])
+  L -- yes --> T["time = (target - pos) / spd"]
+  T --> Q{"stack empty or time > stack[-1]?"}
+  Q -- yes --> A["stack.append(time) — slower, so it starts a new fleet"]
+  Q -- no --> B["discard — it catches the car ahead and joins that fleet"]
+  A --> L
+  B --> L
+```
+
 ## Solution

 ```py
@@ -45,3 +61,7 @@ class Solution:

        return len(stack)
 ```
+
+## Explanation
+
+<YouTube id="Pr6T-3yB9RM" title="Car Fleet - Leetcode 853 - Python" />
@@ -0,0 +1,3 @@
+import { defineMeta } from "blume";
+
+export default defineMeta({ title: "Monotonic Stack", order: 7 });
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Medium'
 ---

-<Badge variant="accent">Prefix Sum</Badge>
+<Badge variant="accent">Array</Badge>

 ::::warning
 You must write an algorithm that runs in O(n) time and without using the division operation.
@@ -27,6 +27,17 @@ Can you solve the problem in O(1) extra space complexity? (The output array does
 - `-30 <= nums[i] <= 30`
 - The input is generated such that `answer[i]` is guaranteed to fit in a 32-bit integer.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["productExceptSelf(nums)"]) --> I["answer = ones(n), rightArr = ones(n)"]
+  I --> P1["Pass 1 — left to right: answer[i] = nums[i-1] * answer[i-1]"]
+  P1 --> P2["Pass 2 — right to left: rightArr[i] = nums[i+1] * rightArr[i+1]"]
+  P2 --> P3["Pass 3: answer[i] *= rightArr[i]"]
+  P3 --> E(["return answer — no division needed"])
+```
+
 ## Solution

 ```js
@@ -51,3 +62,7 @@ var productExceptSelf = function(nums) {
  return answer;
 };
 ```
+
+## Explanation
+
+<YouTube id="bNvIQI2wAjk" title="Product of Array Except Self - Leetcode 238 - Python" />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Easy'
 ---

-<Badge variant="accent">Prefix Sum</Badge>
+<Badge variant="accent">Array</Badge>

 ### Example 1:
 - Input: `["NumArray", "sumRange", "sumRange", "sumRange"]` `[[[-2, 0, 3, -5, 2, -1]], [0, 2], [2, 5], [0, 5]]`
@@ -20,6 +20,21 @@ sidebar:
 - `0 <= left <= right < nums.length`
 - At most `10^4` calls will be made to `sumRange`.

+## Approach
+
+```mermaid
+flowchart TD
+  subgraph build["constructor(nums) — O(n) once"]
+    A["prefix = [0]"] --> B{"more n in nums?"}
+    B -- yes --> C["prefix.push(last + n)"]
+    C --> B
+  end
+  build --> Q
+  subgraph Q["sumRange(left, right) — O(1) per call"]
+    D(["return prefix[right + 1] - prefix[left]"])
+  end
+```
+
 ## Solution

 ```js
@@ -50,3 +65,7 @@ class NumArray {
 * var param_1 = obj.sumRange(left,right)
 */
 ```
+
+## Explanation
+
+<YouTube id="2pndAmo_sMA" title="Range Sum Query Immutable - Leetcode 303 - Python" />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Medium'
 ---

-<Badge variant="accent">Prefix Sum</Badge>
+<Badge variant="accent">Array</Badge>

 ### Example 1:
 - Input: `nums = [1,1,1], k = 2`
@@ -22,6 +22,21 @@ sidebar:
 - `-1000 <= nums[i] <= 1000`
 - `-10^7 <= k <= 10^7`

+## Approach
+
+```mermaid
+flowchart TD
+  S(["subarraySum(nums, k)"]) --> I["map = {0: 1} — the empty prefix; sum = 0, count = 0"]
+  I --> L{"more n in nums?"}
+  L -- no --> E(["return count"])
+  L -- yes --> A["sum += n"]
+  A --> Q{"map has sum - k?"}
+  Q -- yes --> C["count += map.get(sum - k) — every earlier prefix that closes a window"]
+  Q -- no --> W["map.set(sum, count of sum + 1)"]
+  C --> W
+  W --> L
+```
+
 ## Solution

 ```js
@@ -49,3 +64,7 @@ var subarraySum = function(nums, k) {
  return count
 };
 ```
+
+## Explanation
+
+<YouTube id="fFVZt-6sgyo" title="Subarray Sum Equals K - Prefix Sums - Leetcode 560 - Python" />
@@ -0,0 +1,3 @@
+import { defineMeta } from "blume";
+
+export default defineMeta({ title: "Prefix Sum", order: 2 });
@@ -1,9 +0,0 @@
---
-title: Progress
-description: Live SRS campaign dashboard — phases, ladder, review queue, gates, and recent attempts, fetched from the SRS Worker.
---
-
-Live view of the 56-day campaign. Everything below is fetched client-side
-from the SRS Worker, so it is always current — no rebuild required.
-
-<ProgressDashboard />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Easy'
 ---

-<Badge variant="accent">Sliding Window</Badge>
+<Badge variant="accent">Array</Badge>

 ### Example 1:
 - Input: `prices = [7,1,5,3,6,4]`
@@ -23,6 +23,24 @@ sidebar:
 - `1 <= prices.length <= 10^5`
 - `0 <= prices[i] <= 10^4`

+## Approach
+
+::::warning
+The diagram traces the code below literally. That code does not run: `curr` and `ans` are never initialised, so the first `curr > left` test raises `NameError`, and `left` holds a price rather than an index, so `prices[left]` indexes by value.
+::::
+
+```mermaid
+flowchart TD
+  S(["maxProfit(prices)"]) --> I["left = min(prices)"]
+  I --> F{"more right in range(len(prices))?"}
+  F -- no --> E(["return ans"])
+  F -- yes --> W{"curr > left?"}
+  W -- yes --> P["curr -= prices[left]; left += 1"]
+  P --> W
+  W -- no --> M["ans = max(ans, curr)"]
+  M --> F
+```
+
 ## Solution

 ```py
@@ -36,3 +54,7 @@ class Solution:
            ans = max(ans, curr)
        return ans
 ```
+
+## Explanation
+
+<YouTube id="1pkOgXD63yU" title="Sliding Window: Best Time to Buy and Sell Stock - Leetcode 121 - Python" />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Medium'
 ---

-<Badge variant="accent">Sliding Window</Badge>
+<Badge variant="accent">Hash Table</Badge>

 ### Example 1:
 - Input: `s = "abcabcbb"`
@@ -28,6 +28,21 @@ sidebar:
 - `0 <= s.length <= 10^5`
 - `s` consists of English letters, digits, symbols and spaces.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["lengthOfLongestSubstring(s)"]) --> I["left = 0, ans = 0, window = set()"]
+  I --> L{"more (right, c) in s?"}
+  L -- no --> E(["return ans"])
+  L -- yes --> W{"c already in window?"}
+  W -- yes --> P["window.remove(s[left]); left += 1 — shrink until c is free"]
+  P --> W
+  W -- no --> A["window.add(c)"]
+  A --> M["ans = max(ans, right - left + 1)"]
+  M --> L
+```
+
 ## Solution

 ```py
@@ -45,3 +60,7 @@ class Solution:

        return ans
 ```
+
+## Explanation
+
+<YouTube id="wiGpQwVHdE0" title="Longest Substring Without Repeating Characters - Leetcode 3 - Python" />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Medium'
 ---

-<Badge variant="accent">Sliding Window</Badge>
+<Badge variant="accent">Hash Table</Badge>

 ### Example 1:
 - Input: `s = "ABAB", k = 2`
@@ -24,6 +24,21 @@ sidebar:
 - `s` consists of only uppercase English letters.
 - `0 <= k <= s.length`

+## Approach
+
+```mermaid
+flowchart TD
+  S(["characterReplacement(s, k)"]) --> I["count = {}, res = 0, l = 0, maxF = 0"]
+  I --> L{"more (r, c) in s?"}
+  L -- no --> E(["return res"])
+  L -- yes --> A["count[c] += 1; maxF = max(maxF, count[c])"]
+  A --> W{"(r - l + 1) - maxF > k? — too many chars to replace"}
+  W -- yes --> P["count[s[l]] -= 1; l += 1 — shrink from the left"]
+  P --> W
+  W -- no --> M["res = max(res, r - l + 1)"]
+  M --> L
+```
+
 ## Solution

 ```py
@@ -45,3 +60,7 @@ class Solution:
            res = max(res, r - l + 1)
        return res
 ```
+
+## Explanation
+
+<YouTube id="gqXU1UyA8pk" title="Longest Repeating Character Replacement - Leetcode 424 - Python" />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Medium'
 ---

-<Badge variant="accent">Sliding Window</Badge>
+<Badge variant="accent">Hash Table</Badge>

 ### Example 1:
 - Input: `s1 = "ab", s2 = "eidbaooo"`
@@ -22,6 +22,24 @@ sidebar:
 - `1 <= s1.length, s2.length <= 10^4`
 - `s1` and `s2` consist of lowercase English letters.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["checkInclusion(s1, s2)"]) --> G{"k = len(s1) > len(s2)?"}
+  G -- yes --> X(["return False"])
+  G -- no --> I["need = Counter(s1), window = Counter()"]
+  I --> L{"more (right, c) in s2?"}
+  L -- no --> E(["return False"])
+  L -- yes --> A["window[c] += 1"]
+  A --> B{"right >= k? — window is now longer than k"}
+  B -- yes --> C["drop s2[right - k] from window, deleting the key at 0"]
+  B -- no --> Q
+  C --> Q{"window == need?"}
+  Q -- yes --> R(["return True"])
+  Q -- no --> L
+```
+
 ## Solution

 ```py
@@ -49,3 +67,7 @@ class Solution:

        return False
 ```
+
+## Explanation
+
+<YouTube id="UbyhOgBN834" title="Permutation in String - Leetcode 567 - Python" />
@@ -1,6 +1,7 @@
 ---
 title: Sliding Window
 description: 'Sliding window is a specialized two-pointer family where the pointers define a contiguous interval and you maintain information about that entire interval as it moves.'
+hidden: true
 ---

 :::warning
@@ -0,0 +1,3 @@
+import { defineMeta } from "blume";
+
+export default defineMeta({ title: "Sliding Window", order: 4 });
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Medium'
 ---

-<Badge variant="accent">Stack</Badge>
+<Badge variant="accent">Array</Badge>

 ### Example 1:
 - Input: `tokens = ["2","1","+","3","*"]`
@@ -28,6 +28,21 @@ sidebar:
 - `1 <= tokens.length <= 10^4`
 - `tokens[i]` is either an operator: "+", "-", "*", or "/", or an integer in the range [-200, 200].

+## Approach
+
+```mermaid
+flowchart TD
+  S(["evalRPN(tokens)"]) --> I["stack = [], operations = + - * /"]
+  I --> L{"more token c?"}
+  L -- no --> E(["return stack[0]"])
+  L -- yes --> Q{"c is an operator?"}
+  Q -- no --> N["stack.append(int(c))"]
+  N --> L
+  Q -- yes --> P["y = stack.pop(), x = stack.pop() — order matters"]
+  P --> C["stack.append(operations[c](x, y))"]
+  C --> L
+```
+
 ## Solution

 ```py
@@ -53,3 +68,7 @@ class Solution:

        return stack[0]
 ```
+
+## Explanation
+
+<YouTube id="iu0082c4HDE" title="Evaluate Reverse Polish Notation - Leetcode 150 - Python" />
@@ -23,6 +23,28 @@ You must implement a solution with O(1) time complexity for each function.
 - Methods pop, top and getMin operations will always be called on non-empty stacks.
 - At most 3 * 10^4 calls will be made to push, pop, top, and getMin.

+## Approach
+
+```mermaid
+flowchart TD
+  subgraph state["two parallel stacks — same depth, always"]
+    A["stack — the values"]
+    B["minStack — the minimum as of that depth"]
+  end
+  state --> P
+  subgraph P["push(value)"]
+    P1["stack.append(value)"] --> P2["minStack.append(min(value, minStack[-1]))"]
+  end
+  state --> O
+  subgraph O["pop()"]
+    O1["pop both stacks together"]
+  end
+  state --> R
+  subgraph R["top() / getMin() — O(1)"]
+    R1(["stack[-1] / minStack[-1]"])
+  end
+```
+
 ## Solution

 ```py
@@ -54,3 +76,7 @@ class MinStack:
 # param_3 = obj.top()
 # param_4 = obj.getMin()
 ```
+
+## Explanation
+
+<YouTube id="qkLl7nAwDPo" title="Design Min Stack - Amazon Interview Question - Leetcode 155 - Python" />
@@ -6,7 +6,7 @@ sidebar:
  badge: 'Easy'
 ---

-<Badge variant="accent">Stack</Badge>
+<Badge variant="accent">String</Badge>

 ### Example 1:
 - Input: `s = "()"`
@@ -33,6 +33,24 @@ sidebar:
 - `1 <= s.length <= 10^4`
 - `s` consists of parentheses only '()[]{}'.

+## Approach
+
+```mermaid
+flowchart TD
+  S(["isValid(s)"]) --> I["stack = [], pairs = closer to opener"]
+  I --> L{"more char c?"}
+  L -- no --> Z{"stack empty?"}
+  Z -- yes --> T(["return True"])
+  Z -- no --> U(["return False — unclosed openers left"])
+  L -- yes --> Q{"c is a closer?"}
+  Q -- no --> A["stack.append(c) — it is an opener"]
+  A --> L
+  Q -- yes --> M{"stack non-empty and pairs[c] == stack[-1]?"}
+  M -- yes --> P["stack.pop() — matched"]
+  P --> L
+  M -- no --> F(["return False — mismatch"])
+```
+
 ## Solution

 ```py
@@ -52,3 +70,7 @@ class Solution:

        return True if not stack else False
 ```
+
+## Explanation
+
+<YouTube id="WTzjTskDFMg" title="Valid Parentheses - Stack - Leetcode 20 - Python" />
Author	SHA1	Message	Date
Prad Nukala	f0f479d6cf	feat: add lc 46, 39, 78	2026-09-24 14:36:19 -04:00
Prad Nukala	ac4dc654fc	feat(work): add solution for LeetCode 323 number of connected components problem	2026-09-23 15:40:39 -04:00
Prad Nukala	273148c08b	feat: add lc 684	2026-09-23 15:22:26 -04:00
Prad Nukala	7960dee431	docs(docs): adjust default zoom to 1.6× for improved label readability	2026-09-22 15:10:30 -04:00
Prad Nukala	0009075138	feat(work): implement Course Schedule II solution	2026-09-22 15:02:31 -04:00
Prad Nukala	ca73d3235c	feat(work): add Course Schedule II solution and clean up Course Schedule I implementation	2026-09-22 14:24:38 -04:00
Prad Nukala	97b62f95c9	docs(docs): detail custom MDX components and islands in docs overview	2026-09-22 14:11:37 -04:00
Prad Nukala	1a440e55ce	docs(docs): add Carousel component and embed in topological‑sort page, replace static spine image with interactive component	2026-09-22 14:11:33 -04:00
Prad Nukala	fd3b1952b1	docs(docs): add Topological Sort documentation page with example code and images	2026-09-22 13:28:23 -04:00
Prad Nukala	82c26e2b6c	docs: update README URLs and fix sync bullet formatting	2026-09-22 13:06:27 -04:00
Prad Nukala	f78ecb2bd4	docs(docs): add default order 0 to Array meta	2026-09-22 13:06:23 -04:00
Prad Nukala	b3d3c5fbc9	refactor(cli): default group order to 0 and clarify meta comment	2026-09-22 13:06:20 -04:00
Prad Nukala	99a9248368	chore(githooks): add pre-commit hook to sync docs and stage generated pages	2026-09-22 13:01:32 -04:00
Prad Nukala	2a925f6a9a	docs(githooks): replace pre-push hook with pre-commit and update documentation	2026-09-22 13:01:28 -04:00
Prad Nukala	7d57faa958	feat(work): add solution stub for Course Schedule (207)	2026-09-22 13:01:22 -04:00
Prad Nukala	26085cc027	docs(docs): add Word Ladder and Rotting Oranges documentation pages	2026-09-22 12:59:21 -04:00
Prad Nukala	216152d946	init: hooks	2026-09-22 12:59:13 -04:00
Prad Nukala	d0ace85bbc	docs(agents): add .githooks entry describing git hook enforcement and pre‑push sync check	2026-09-22 12:59:00 -04:00
Prad Nukala	5b99a6a589	feat(work): add Word Ladder and Rotting Oranges solutions	2026-09-22 12:58:56 -04:00
Prad Nukala	8446623607	chore(config): add prepare script for git hooks	2026-09-22 12:58:54 -04:00
Prad Nukala	3a1df27d30	docs(readme): replace static chart images with live SRS API badges and update description.	2026-09-22 11:58:58 -04:00
Prad Nukala	ae1d1ceede	feat(work): implement Pacific Atlantic Water Flow solution	2026-09-21 18:05:35 -04:00
Prad Nukala	f5f31ba1c3	docs(docs): remove array order meta and add solution implementation for Pacific Atlantic Water Flow	2026-09-21 18:05:30 -04:00
Prad Nukala	d1c99c307e	docs(docs): add order to array meta	2026-09-21 17:41:19 -04:00
Prad Nukala	021b1fd06e	docs(agents): update docs for topic folder naming and sync behavior	2026-09-21 17:37:31 -04:00
Prad Nukala	edd37ed0e6	docs: rename category folders to remove parentheses and update heap meta title	2026-09-21 17:37:26 -04:00
Prad Nukala	c2a8a31ede	chore(config): remove Reference tab from navigation	2026-09-21 17:37:22 -04:00
Prad Nukala	891a1f0c5a	refactor(cli): clarify topic folder handling and remove unused groupFolder helper	2026-09-21 17:37:19 -04:00
Prad Nukala	2e5d991d47	chore(config): bump multiple package versions in bun.lock	2026-09-21 17:16:31 -04:00
Prad Nukala	73d63c2565	docs(docs): clarify apps/docs layout and bump blume to ^1.7.3	2026-09-21 17:16:18 -04:00
Prad Nukala	cafc8a4c25	feat(work): add solutions for number-of-islands, pacific-atlantic-water-flow, and clone-graph problems	2026-09-21 17:16:13 -04:00
Prad Nukala	7c5061dcd1	docs(docs): add meta files and reorganize topics with updated badges and directory structure	2026-09-21 17:16:10 -04:00
Prad Nukala	f9e69960c4	feat(cli): add topic grouping with meta.ts generation in sync script	2026-09-21 17:15:56 -04:00
Prad Nukala	0c12df3eba	docs(readme): update LeetCode profile link to progress page	2026-09-21 15:55:59 -04:00
Prad Nukala	bfb53c14e8	docs(readme): wrap LeetCode badge in profile link	2026-09-21 15:54:29 -04:00
Prad Nukala	2c30b3d017	feat(problem-approach): add skill for generating approach diagrams	2026-09-21 15:53:32 -04:00
Prad Nukala	1084410832	docs(readme): add videos command and update schedule dates and badges	2026-09-21 15:53:28 -04:00
Prad Nukala	76c9c77307	feat(work): add Number of Islands solution	2026-09-21 15:53:24 -04:00
Prad Nukala	23eb71d387	docs(docs): add Approach diagrams and Explanation sections to LeetCode problem documentation	2026-09-21 15:53:21 -04:00
Prad Nukala	b8c779903f	feat(config): add videos script to package.json	2026-09-21 15:53:18 -04:00
Prad Nukala	5059eb72cb	feat(cli): add videos script for NeetCode embed lookup and CLI command	2026-09-21 15:53:15 -04:00
Prad Nukala	204e178fa4	feat(work): add solution for LeetCode 211 design add and search words data structure	2026-09-21 12:50:53 -04:00
Prad Nukala	61dc19fb29	docs(docs): add MDX page for LeetCode 211 design problem	2026-09-21 12:50:51 -04:00
Prad Nukala	5c644ca050	docs(schedule): clarify schedule JSON format, gate problem handling, and update week dates in README	2026-09-21 12:14:37 -04:00
Prad Nukala	fb2d4c88c2	feat(work): add solution for LeetCode 208 Implement Trie (Prefix Tree)	2026-09-21 12:14:33 -04:00
Prad Nukala	1fa0731ea7	docs(docs): add MDX page for LeetCode 208 Implement Trie (Prefix Tree)	2026-09-21 12:14:29 -04:00
Prad Nukala	86107d5ef7	feat(api): add structured schedule plan with topics, problems, gate, sweep and reserved problem handling	2026-09-21 12:14:25 -04:00
Prad Nukala	00f44a1c47	docs(docs): add MDX page for Merge k Sorted Lists problem	2026-09-14 12:50:08 -04:00
Prad Nukala	24312437eb	feat(work): add solution for LeetCode 23 Merge k Sorted Lists	2026-09-14 12:49:55 -04:00
Prad Nukala	4c9c8f39ae	feat(work): add solution for LeetCode 973 K Closest Points to Origin	2026-09-14 12:37:15 -04:00
Prad Nukala	88ff63f5c4	docs(docs): add MDX page for LeetCode 973 K Closest Points to Origin	2026-09-14 12:37:11 -04:00
Prad Nukala	2066856d15	feat(work): add solution for LeetCode 215 Kth Largest Element in an Array	2026-09-14 12:21:55 -04:00
Prad Nukala	393d72c9fe	docs(docs): add MDX page for LeetCode 215 Kth Largest Element in an Array	2026-09-14 12:21:51 -04:00
Prad Nukala	6c18301aa6	feat(work): add Lowest Common Ancestor of BST solution	2026-09-05 16:33:03 -04:00
Prad Nukala	02f6a20cc1	docs(docs): add documentation for 235 Lowest Common Ancestor of a Binary Search Tree	2026-09-05 16:32:59 -04:00
Prad Nukala	9288aaa152	feat(work): add Kth Smallest Element in a BST solution	2026-09-05 16:28:21 -04:00
Prad Nukala	d5ea71dbc1	docs(docs): add MDX page for “230. Kth Smallest Element in a BST”	2026-09-05 16:28:18 -04:00
Prad Nukala	3e1a66eb37	feat(work): add LeetCode solutions for 217, 271, and 98	2026-09-05 13:37:53 -04:00
Prad Nukala	9a2b80949b	docs(docs): add 271 Encode/Decode Strings and 98 Validate BST pages	2026-09-05 13:37:42 -04:00
Prad Nukala	883dc69dba	docs(docs): update index title from “Progress” to “Overview”	2026-09-04 16:49:23 -04:00
Prad Nukala	e3f0b5420a	docs(docs): replace index with live progress dashboard and delete redundant progress page	2026-09-04 16:46:50 -04:00
Prad Nukala	987eb7ab5b	feat(work): add solution for LeetCode 1448 Count Good Nodes in Binary Tree	2026-09-04 16:44:07 -04:00
Prad Nukala	e4118fccd3	docs(docs): add MDX page for LeetCode 1448 Count Good Nodes in Binary Tree problem	2026-09-04 16:44:04 -04:00
Prad Nukala	95db4ca46c	feat(work): add solution for Binary Tree Right Side View problem	2026-09-04 11:48:03 -04:00
Prad Nukala	0cc7ca607e	docs(docs): add documentation for Binary Tree Right Side View problem	2026-09-04 11:47:59 -04:00
Prad Nukala	71ec39fe34	feat(work): add solution for Binary Tree Level Order Traversal (LC 102)	2026-09-04 11:33:20 -04:00
Prad Nukala	51decd23bd	docs(docs): add documentation for Binary Tree Level Order Traversal (LeetCode 102)	2026-09-04 11:33:15 -04:00
Prad Nukala	90082518a6	feat(work): add solution for Lowest Common Ancestor of a Binary Tree	2026-09-03 14:17:54 -04:00
Prad Nukala	e10a913546	docs(docs): add 236 Lowest Common Ancestor of a Binary Tree documentation	2026-09-03 14:17:51 -04:00
Prad Nukala	fec94ed636	feat(work): add solution for Invert Binary Tree (LC 226)	2026-09-03 14:01:06 -04:00
Prad Nukala	30273d0030	docs(docs): add Invert Binary Tree documentation	2026-09-03 14:01:02 -04:00
Prad Nukala	50299defef	feat(work): add Maximum Depth of Binary Tree solution	2026-09-03 13:52:57 -04:00
Prad Nukala	c843be855e	feat(docs): add Maximum Depth of Binary Tree documentation	2026-09-03 13:52:54 -04:00
Prad Nukala	1e0e03daaa	feat(work): add Set Matrix Zeroes solution	2026-09-02 16:52:29 -04:00
Prad Nukala	7b182206dd	docs(docs): add documentation for Set Matrix Zeroes problem	2026-09-02 16:52:26 -04:00
Prad Nukala	faaaf52f85	feat(work): add Rotate Image and Spiral Matrix solutions	2026-09-02 16:49:41 -04:00
Prad Nukala	090f3d6719	docs(docs): add new problem pages for Rotate Image, Spiral Matrix, and Add Two Numbers	2026-09-02 16:49:39 -04:00
Prad Nukala	a3ae16ad96	feat(work): add Group Anagrams solution	2026-09-01 15:56:02 -04:00
Prad Nukala	9faa0b0337	feat(work): implement correct addTwoNumbers algorithm for linked list addition	2026-09-01 14:39:45 -04:00
Prad Nukala	cc29304903	docs(agents): clarify sync.ts contract description for solution section ownership	2026-09-01 14:22:01 -04:00
Prad Nukala	c50b600103	feat(work): add Add Two Numbers solution	2026-09-01 14:21:57 -04:00
Prad Nukala	a86d72c7db	docs(docs): add Approach and Explanation sections with mermaid diagrams to problem pages	2026-09-01 14:21:54 -04:00
Prad Nukala	e59d78e571	feat(cli): preserve custom sections after solution during docs sync	2026-09-01 14:21:50 -04:00
Prad Nukala	a93bb068b6	fix(work): correct node insertion and eviction order in LRUCache	2026-09-01 13:59:03 -04:00
Prad Nukala	53b2d5ec8c	docs(docs): clarify LRUCache insertion order in example MDX	2026-09-01 13:59:01 -04:00
Prad Nukala	5dda64e41e	feat(work): add LRU Cache solution	2026-09-01 13:31:34 -04:00
Prad Nukala	d798871fbc	docs(docs): add LRU Cache problem documentation	2026-09-01 13:31:31 -04:00
Prad Nukala	bb7324fe8c	docs(AGENTS): add terminal dashboard info and expand documentation for stats, work layout, and tooling preferences	2026-08-31 18:56:57 -04:00
Prad Nukala	d13859d31e	refactor(docs): rename recent stats to heat and update dashboard rendering	2026-08-31 18:56:52 -04:00
Prad Nukala	664ea534ed	feat(config): add stats script to package.json	2026-08-31 18:56:46 -04:00
Prad Nukala	b86f36daf1	feat(cli): add terminal stats dashboard and solution detection utilities	2026-08-31 18:56:42 -04:00
Prad Nukala	7bae7b2920	feat(api): add concept temperature bands and expose in stats	2026-08-31 18:56:40 -04:00
Prad Nukala	64173857e7	feat(work): add new LeetCode solution files for 1426, 1832, 344, and 15	2026-08-31 16:50:30 -04:00
Prad Nukala	002c2c9c97	feat(work): add solutions for LeetCode 1365 and 268 problems	2026-08-31 16:36:29 -04:00
Prad Nukala	fa8deac963	feat(work): add solution for LeetCode 387 First Unique Character in a String	2026-08-31 16:21:47 -04:00
Prad Nukala	ef8d2b41e1	docs(todos): add review backlog planning markdown	2026-08-31 14:57:38 -04:00