feat: Add car fleet

.claude/skills/campaign-issues/SKILL.md

@@ -0,0 +1,175 @@
+---
+name: campaign-issues
+description: Create GitHub issues for this LeetCode campaign — individual problem issues and the topic issues that own them — using the repo's exact title, label, milestone, body, and sub-issue conventions. Use when asked to add/create an issue for a LeetCode problem or a topic, attach a problem to its topic, or backfill the issue catalog.
+---
+
+# Campaign issues
+
+The GitHub issue tracker **is the campaign catalog**: ~24 `topic` issues each own a
+set of `problem` sub-issues. The Worker reconciles this catalog into D1 nightly
+(`/admin/reconcile`), so a correctly-labelled issue is all that's needed — the SRS,
+digest, charts, and Project mirror pick it up automatically. Get the labels/body shape
+right; everything downstream is reconciliation.
+
+All commands use `gh` against `prdlk/leetcode` and fall back to `gh auth token`.
+
+## Problem issue
+
+One issue per LeetCode problem, attached as a sub-issue of its topic.
+
+- **Title:** `LC <num> · <Title>` — the LC number and the problem's name, nothing
+  else. Separator is a middle dot `·` (U+00B7) with single spaces. Difficulty and
+  set are **labels only**; never repeat them in the title (they used to be a `·
+  Hard · core` suffix — that is gone, and `catalog.ts` no longer parses it).
+- **Labels** (four, all required — these are what the Worker reads):
+  - `problem` — marks the sub-issue as curriculum; without it the reconcile skips it.
+  - `diff:easy` | `diff:medium` | `diff:hard` — LeetCode's own rating, do not invent.
+  - `set:core` | `set:optional` | `set:deferred` — see set semantics below.
+  - `phase:N` — the **topic's** phase (same phase label the topic issue carries).
+- **Milestone:**
+  - `core` / `optional` → the topic's phase milestone.
+  - `deferred` → `Review — Transfer & Mocks` (deferred Hards are parked until week 8).
+  - Pass the milestone **title**, not its number, to `--milestone`.
+- **Body** (exact shape; `<slug>` is the LeetCode URL slug, `<topic-issue>` the owning topic's number):
+
+  ```
+  https://leetcode.com/problems/<slug>/
+
+  - **Topic:** #<topic-issue>
+  - **Set:** <core|optional|deferred>
+  - **Language:** Python
+
+  ### Close-out (paste before closing this issue)
+  - Time: O(?)
+  - Space: O(?)
+  - Edge case traced:
+  - First attempt passed: yes / no
+  - Miss note:
+  ```
+
+Create it:
+
+```sh
+gh issue create --repo prdlk/leetcode \
+  --title "LC 42 · Trapping Rain Water" \
+  --label problem --label set:core --label diff:hard --label phase:1 \
+  --milestone "Phase I — Linear Structures" \
+  --body "https://leetcode.com/problems/trapping-rain-water/
+
+- **Topic:** #9
+- **Set:** core
+- **Language:** Python
+
+### Close-out (paste before closing this issue)
+- Time: O(?)
+- Space: O(?)
+- Edge case traced:
+- First attempt passed: yes / no
+- Miss note:"
+```
+
+Then **attach it as a sub-issue of the topic** (this is a separate API call — creating
+the issue does not link it). The sub-issues API wants the child's internal **id**, not
+its issue number, and the field is integer-typed, so use `-F` (not `-f`):
+
+```sh
+child_id=$(gh api repos/prdlk/leetcode/issues/211 --jq '.id')
+gh api --method POST repos/prdlk/leetcode/issues/9/sub_issues -F sub_issue_id=$child_id
+# verify
+gh api repos/prdlk/leetcode/issues/9/sub_issues --jq '.[] | "\(.number)  \(.title)"'
+```
+
+## Topic issue
+
+One issue per curriculum topic; owns the problem sub-issues.
+
+- **Title:** `Topic NN — <Name>` (zero-padded number, em dash, e.g. `Topic 07 — Monotonic Stack`).
+- **Labels:** `topic`, `phase:N`.
+- **Milestone:** that phase's milestone (title, not number).
+- **Body:**
+
+  ```
+  **Scheduled:** <Weekday Mon DD, YYYY>
+  **Phase:** <Roman> — <Phase name> (Week N)
+  **Builds on:** Topic <n>
+
+  ### Trigger
+  TODO
+
+  ### Invariant
+  TODO
+
+  ### Trap
+  TODO
+
+  ### Close-out ritual
+  Before every submit, out loud:
+  1. State time and space complexity.
+  2. Trace one edge case.
+
+  ### Day rules
+  - 90-minute coding cap. Stop at the cap, even mid-problem.
+  - Core problems first. Optional only if the core is done.
+  - Finished early? Do this topic's optional set. Do not start tomorrow.
+  ```
+
+`Scheduled` should come from the calendar of record — `apps/api/data/schedule.json`
+(day → topic issue) — not guessed.
+
+## Reference tables
+
+Set semantics (from the label descriptions):
+
+| set | meaning | milestone |
+|---|---|---|
+| `core` | must solve on the topic day | topic's phase milestone |
+| `optional` | solve only if the clock allows | topic's phase milestone |
+| `deferred` | a Hard, parked until week 8 | `Review — Transfer & Mocks` |
+
+Milestones (`--milestone` takes the **title**):
+
+| # | title |
+|---|---|
+| 1 | Phase I — Linear Structures |
+| 2 | Phase II — Nodal & Grid |
+| 3 | Phase III — Hierarchical |
+| 4 | Phase IV — Relational |
+| 5 | Phase V — Decision Space |
+| 6 | Review — Transfer & Mocks |
+
+Find a topic issue and confirm its phase before creating problems under it:
+
+```sh
+gh issue list --repo prdlk/leetcode --label topic --state all --limit 30 \
+  --json number,title,labels,milestone
+```
+
+## Gotchas
+
+- **Check the LC number is not already in the curriculum BEFORE creating.** D1 keys
+  `problems` on `lc_number` (`ON CONFLICT(lc_number)`), so two issues for one LC number
+  silently fight over a single row — whichever the reconcile walks last wins, and the
+  loser's issue number ends up orphaned in D1. A problem can legitimately move topic or
+  set; that is a relabel/re-parent of the *existing* issue, never a second issue:
+
+  ```sh
+  gh issue list --repo prdlk/leetcode --label problem --state all --limit 300 \
+    --search "LC 42" --json number,title,labels,state
+  ```
+
+  If a row already exists, change the existing issue instead: swap its `set:*` label,
+  move the sub-issue to the new topic, and fix its milestone. Then reconcile the board
+  (`project-fields` skill). If a duplicate did get created and one is deleted, the
+  surviving issue number and set only reach D1 on the next catalog reconcile — SRS
+  columns (`stage`, `next_review`) are preserved, since they are the real attempt history.
+
+- **`-F` vs `-f`:** `sub_issue_id` must be an integer; `-f` sends a string and the API
+  rejects it with `is not of type integer`.
+- **id, not number:** the sub-issue payload wants the child's `.id` (a large opaque
+  integer), never its display `#number`.
+- **`--milestone` wants the title string**, not the numeric id (`'1' not found` otherwise).
+- **Reconcile, don't react:** don't edit D1 or the Project by hand — a correct issue is
+  reconciled into SRS state on the nightly cron. Solutions landing in `work/` are what
+  close problem issues (via `close-solved.yml`), which then roll up to close topics.
+- **Difficulty is LeetCode's**, `set` is our scheduling choice — they're independent. A
+  Hard can be `core` if we deliberately schedule it (vs the usual `deferred`).

.claude/skills/project-fields/SKILL.md

@@ -0,0 +1,108 @@
+---
+name: project-fields
+description: Reconcile the "Interview Prep" GitHub Project (user project #2) fields — Set and Difficulty projected from issue labels — and understand which fields the Worker mirrors vs which live only in D1. Use when asked to sync/fix/backfill Project fields, when Project columns look empty or stale, or when adding a field to the project.
+---
+
+# Interview Prep Project fields
+
+The user-level Project **"Interview Prep"** (`prdlk` project #2, id `PVT_kwHOADoNjs4BhV1a`)
+is a *board over* the issue catalog, never a source of truth. Every field is a projection
+of something owned elsewhere.
+
+## Field ownership — the rule that matters
+
+| Field | Owner | Written by |
+|---|---|---|
+| `Set` (Core/Optional/Deferred) | issue label `set:*` | `sync-project-fields`, last step of `close-solved.yml` |
+| `Difficulty` (Easy/Medium/Hard) | issue label `diff:*` | `sync-project-fields`, last step of `close-solved.yml` |
+| `Target Date` | D1 `next_review` | Worker mirror, per log (`apps/api/src/mirror.ts`) |
+| `Status`, `Labels`, `Milestone`, `Parent issue`, … | GitHub built-ins | GitHub |
+
+**SRS stage and first-attempt result are deliberately NOT Project fields.** They live only
+in D1; the charts, digest and `/api/stats` read them from there. Duplicating them into
+Project single-selects bought nothing but drift and two extra writes per log. Do not re-add
+them to the project, and do not re-add `setStage`/`setFirstAttempt` to `mirror.ts`.
+
+## Reconciling Set / Difficulty
+
+`apps/cli/sync-project-fields.ts` recomputes every desired value from the issue labels and
+writes only what differs — same reconcile-don't-react shape as `close-solved`.
+
+**In CI it runs itself.** It is the final step of `.github/workflows/close-solved.yml`, so
+any push under `work/` reconciles the board along with the issues and D1. The step is
+`continue-on-error: true` and skips cleanly when `PROJECT_PAT` is unset — D1 and the issues
+are the truth, so a Project failure must never fail a run that already closed issues and
+logged the solve. It honours the workflow's `dry_run` input via `DRY_RUN=true`.
+
+Run it by hand when issues changed without a `work/` push (e.g. you just filed a problem
+issue, or relabelled one):
+
+```sh
+# needs a token with the `project` scope — neither `gh auth token` nor GITHUB_TOKEN has it.
+export GH_TOKEN=$(grep '^GH_PAT=' apps/api/.dev.vars | cut -d= -f2-)
+
+bun run sync-project-fields -- --dry-run   # or DRY_RUN=1|true; prints the table, writes nothing
+bun run sync-project-fields                # apply
+bun run sync-project-fields                # re-run: must report 0 writes
+```
+
+Invariants the script holds — preserve them in any edit:
+
+- **Only `problem`-labelled items are touched.** Topic, cadence and review issues have no
+  set/difficulty; they are skipped, never written and never cleared.
+- **Idempotent.** A second run reports `0 field value(s)`. That re-run *is* the verification.
+- **Labels are truth.** Never "fix" a field by hand in the UI — fix the label, then reconcile.
+  A hand-edited field is silently reverted on the next run.
+- **Fields and options resolve by NAME**, so re-creating a field in the UI doesn't strand a
+  dead id. A missing field or option is a hard error, not a silent skip.
+- **A problem missing a `set:`/`diff:` label** warns and is left untouched (it means the
+  issue is mislabelled — fix it with the `campaign-issues` skill).
+
+## Auth gotcha
+
+Project v2 needs the `project` scope. The local `gh` login has only
+`gist, read:org, repo, workflow`, so **any** `gh project`/GraphQL project call fails with
+the default token. Use the Worker's `GH_PAT` from `apps/api/.dev.vars` via `GH_TOKEN`
+(never echo it). Symptom of the wrong token: `Resource not accessible by personal access
+token` or an empty `projectsV2.nodes`.
+
+## Inspecting the project
+
+```sh
+export GH_TOKEN=$(grep '^GH_PAT=' apps/api/.dev.vars | cut -d= -f2-)
+
+# fields + single-select options
+gh api graphql -f owner=prdlk -f title="Interview Prep" -f query='
+query($owner: String!, $title: String!) {
+  user(login: $owner) { projectsV2(first: 10, query: $title) { nodes { id title number
+    items(first: 1) { totalCount }
+    fields(first: 30) { nodes {
+      ... on ProjectV2FieldCommon { id name dataType }
+      ... on ProjectV2SingleSelectField { options { id name } } } } } } }
+}' --jq '.data.user.projectsV2.nodes[] | select(.title=="Interview Prep")
+         | .fields.nodes[] | "\(.name)\t\(.dataType)\t\((.options // []) | map(.name) | join(","))"'
+```
+
+Writes use `updateProjectV2ItemFieldValue` with
+`value: { singleSelectOptionId: … }` (dates use `{ date: "YYYY-MM-DD" }`); clearing uses
+`clearProjectV2ItemFieldValue`. An item id is *not* an issue number — resolve it from the
+project's `items` connection (or the issue's `projectItems`), as `mirror.ts` does.
+
+## Adding a new projected field
+
+1. Create the field + options in the Project UI.
+2. Add it to the `FIELDS` table in `apps/cli/sync-project-fields.ts` — one entry mapping
+   label → option name. Nothing else needs to change; resolution, diffing, dry-run and
+   reporting are generic over that table.
+3. Dry-run, apply, re-run to prove idempotency.
+
+If the new field's truth lives in **D1** rather than a label, it belongs in the Worker
+mirror instead (`apps/api/src/mirror.ts`) — but read the ownership rule above first and
+default to *not* mirroring.
+
+## Known trap
+
+`resolve()` in `mirror.ts` throws if a field it asks for is absent, *before* caching
+`info` — so one missing field breaks **every** mirrored write, including `Target Date`,
+and only as a `console.warn`. If Target Date silently stops updating, check that every
+field `resolve()` looks up still exists in the project.

.github/workflows/close-solved.yml

@@ -39,3 +39,25 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           DRY_RUN: ${{ inputs.dry_run }}
           SRS_ADMIN_KEY: ${{ secrets.SRS_ADMIN_KEY }}
+
+      # Project fields are a projection of the issue labels, so the same push
+      # that adds a solution (and any issue filed since the last run) can leave
+      # the board's Set/Difficulty empty. Reconciling here keeps the one
+      # "solution landed" entry point responsible for the whole fan-out; the
+      # script is idempotent, so a run with nothing to do writes nothing.
+      #
+      # Needs the `project` scope, which GITHUB_TOKEN does not have — hence
+      # PROJECT_PAT. Unset = skip, matching SRS_ADMIN_KEY above. Best-effort by
+      # design: D1 and the issues are the truth, so a Project failure must not
+      # fail a run that already closed issues and logged the solve.
+      - name: Reconcile Project Set/Difficulty
+        continue-on-error: true
+        env:
+          GH_TOKEN: ${{ secrets.PROJECT_PAT }}
+          DRY_RUN: ${{ inputs.dry_run }}
+        run: |
+          if [ -z "$GH_TOKEN" ]; then
+            echo "PROJECT_PAT unset — skipping the Project field reconcile"
+            exit 0
+          fi
+          bun apps/cli/sync-project-fields.ts

AGENTS.md

@@ -2,7 +2,7 @@
 
 ## Project Overview
 
-LeetCode interview-prep campaign (8 weeks, 2026-08-17 → Oct 11) run as a repo: solution files under `work/`, a Blume docs site under `apps/docs/`, GitHub Actions that reconcile issues and publish docs, and a Cloudflare Worker (`apps/api/`) that runs the spaced-repetition system (SRS) — daily digest email, one-tap logging, weekly review issues, live charts. Issues model the curriculum: ~24 `topic` issues own ~161 `problem` sub-issues (`set:core|optional|deferred`, `diff:*`); milestones are phases; a user-level GitHub Project ("Interview Prep", #2) mirrors review state.
+LeetCode interview-prep campaign (8 weeks, 2026-08-17 → Oct 11) run as a repo: solution files under `work/`, a Blume docs site under `apps/docs/`, GitHub Actions that reconcile issues and publish docs, and a Cloudflare Worker (`apps/api/`) that runs the spaced-repetition system (SRS) — daily digest email, one-tap logging, weekly review issues, live charts. Issues model the curriculum: ~24 `topic` issues (`Topic NN — Name`) own ~161 `problem` sub-issues titled `LC <num> · <Name>`, with difficulty and set carried **only** by labels (`set:core|optional|deferred`, `diff:*`) — never by the title; milestones are phases; a user-level GitHub Project ("Interview Prep", #2) mirrors review state.
 
 ## Architecture & Data Flow
 
@@ -18,7 +18,8 @@ flowchart LR
   D1 -->|8 AM ET cron| Mail[digest email + one-tap links]
   Mail -->|GET /log| D1
   PI -->|/done webhook| D1
-  D1 -->|best-effort mirror| Proj[GitHub Project fields]
+  D1 -->|best-effort mirror: Target Date only| Proj[GitHub Project fields]
+  GH -->|close-solved.yml sync-project-fields| Proj
   D1 -->|Sat midnight ET| Review[Review — Week N issue]
   D1 --> Charts[SVG charts + /api/stats]
 ```
@@ -26,7 +27,7 @@ flowchart LR
 - **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.
 - **Solving has three doors, one write path.** A digest one-tap, a `/done` comment, and a solution landing in `work/` all end in `logAttempt()`. Closing an issue is *not* one of them: the catalog reconcile never reads issue state, so a `work/` push that skipped the email would otherwise leave `stage='new'` — invisible to the charts, the digest's solved ticks, and the drill/gate pools. Hence `/admin/solved` (`source='commit'`, first solve only, no index needed for idempotency: it refuses any problem past stage `new`). `close-topics` needs no such call — D1 stores no topic completion, only problem rows.
 - **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`). Project fields (`Target Date`, `SRS Stage`, `First Attempt`) are a best-effort mirror (`apps/api/src/mirror.ts`): failures are warnings, never lost D1 writes; topic rows' `Target Date` is never written.
+- **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.
 - **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.
@@ -39,7 +40,7 @@ flowchart LR
 | `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/`, `scripts/import-srs.ts` |
-| `.github/workflows/` | deploy (docs → Pages, Worker → Cloudflare, on every main push), close-solved (issues + `/admin/solved`, needs `SRS_ADMIN_KEY`), close-topics, sync-d1 (issue edits → `/admin/reconcile`) |
+| `.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`) |
 
 ## Development Commands
 
@@ -60,6 +61,9 @@ bun run close-topics -- --dry-run
 # close-solved also pushes the solved set to the Worker; point it at a local
 # one and nothing production is touched:
 SRS_API=http://localhost:8787 SRS_ADMIN_KEY=$LINK_KEY bun run close-solved -- --dry-run
+# Project Set/Difficulty from the issue labels (needs a `project`-scoped token;
+# runs as the last step of close-solved.yml in CI):
+GH_TOKEN=$PROJECT_PAT bun run sync-project-fields -- --dry-run
 bun run api:dev         # wrangler dev on :8787 (local D1); api:test = DST guard tests
 bun run api:deploy      # deploy the Worker by hand (CI also deploys on main pushes)
 curl -X POST -H "Authorization: Bearer $LINK_KEY" \

apps/api/README.md

@@ -16,9 +16,13 @@ Live at `https://srs-api.prdlk.workers.dev`.
   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 GitHub Project mirror (`Target Date`, `SRS Stage`, `First Attempt`)
-  is best-effort: failures log warnings, never block a D1 write. Topic rows
-  are never written.
+- 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 —
+  the charts, digest and `/api/stats` read them from there, so mirroring them
+  into Project single-selects bought only drift. Do not re-add them.
+  `Set`/`Difficulty` are a projection of the issue labels, reconciled by
+  `bun run sync-project-fields` at the repo root, not by this Worker.
 
 ## The ladder
 

apps/api/src/catalog.ts

@@ -4,11 +4,19 @@
  * rows from scratch on every run — re-runs are no-ops. SRS-owned columns
  * (stage, next_review) are NEVER overwritten; defer_until is set once, on
  * first sight of a deferred problem. The Worker never invents catalog rows.
+ *
+ * Difficulty and set come from the `diff:*` / `set:*` LABELS, never from the
+ * issue title — the title carries only the LC number and the problem's name,
+ * 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 { addDays } from "./srs.ts";
 import type { GitHub } from "./github.ts";
 
-const TITLE_RE = /^LC (\d+) · (.+) · (Easy|Medium|Hard) · (core|optional|deferred)$/;
+const TITLE_RE = /^LC (\d+) · (.+)$/;
+
+const DIFFICULTIES = ["easy", "medium", "hard"] as const;
+const SETS = ["core", "optional", "deferred"] as const;
 
 /** Deferred Hards enter the queue from this date, two per day. */
 const DEFER_FROM = "2026-09-28";
@@ -18,6 +26,18 @@ export interface ReconcileReport {
   problems: number;
 }
 
+/** Label names on one issue payload, narrowed with guards rather than cast. */
+function labelNames(value: object): string[] {
+  if (!("labels" in value) || !Array.isArray(value.labels)) return [];
+  const names: string[] = [];
+  for (const label of value.labels) {
+    if (label && typeof label === "object" && "name" in label && typeof label.name === "string") {
+      names.push(label.name);
+    }
+  }
+  return names;
+}
+
 export async function reconcileCatalog(db: D1Database, gh: GitHub): Promise<ReconcileReport> {
   interface TopicIssue {
     number: number;
@@ -62,10 +82,20 @@ export async function reconcileCatalog(db: D1Database, gh: GitHub): Promise<Reco
       if (!raw || typeof raw !== "object") continue;
       if (!("number" in raw) || typeof raw.number !== "number") continue;
       if (!("title" in raw) || typeof raw.title !== "string") continue;
+      const labels = labelNames(raw);
+      if (!labels.includes("problem")) continue; // non-curriculum sub-issue
       const m = raw.title.match(TITLE_RE);
-      if (!m) continue; // non-curriculum sub-issue
+      if (!m) {
+        console.warn(`catalog: #${raw.number} is a problem but not titled "LC <n> · <name>"`);
+        continue;
+      }
+      const difficulty = DIFFICULTIES.find((d) => labels.includes(`diff:${d}`));
+      const set = SETS.find((s) => labels.includes(`set:${s}`));
+      if (!difficulty || !set) {
+        console.warn(`catalog: #${raw.number} is missing a diff:/set: label — skipped`);
+        continue;
+      }
       const lc = Number(m[1]);
-      const set = m[4]!;
       // Two deferred Hards per day from DEFER_FROM, in catalog walk order —
       // applied only when the row is first created (SRS owns it afterwards).
       const defer = set === "deferred" ? addDays(DEFER_FROM, Math.floor(deferredSeen / 2)) : null;
@@ -78,7 +108,7 @@ export async function reconcileCatalog(db: D1Database, gh: GitHub): Promise<Reco
              ON CONFLICT(lc_number) DO UPDATE SET
                issue = ?2, topic_issue = ?3, title = ?4, difficulty = ?5, set_label = ?6`,
           )
-          .bind(lc, raw.number, t.number, m[2]!, m[3]!.toLowerCase(), set, defer),
+          .bind(lc, raw.number, t.number, m[2]!, difficulty, set, defer),
       );
       problems++;
     }

apps/api/src/index.ts

@@ -30,22 +30,16 @@ import { buildStats } from "./stats.ts";
 
 // ── shared side effects after a state write ──────────────────────
 
-/** Project-field half of a mirror; shared by every write path. */
-async function mirrorFields(mirror: Mirror, outcome: LogOutcome): Promise<void> {
-  await mirror.setStage(outcome.issue, outcome.stage);
-  await mirror.setTargetDate(outcome.issue, outcome.next_review);
-  if (outcome.first) await mirror.setFirstAttempt(outcome.issue, outcome.result);
-}
-
 /**
- * Mirror one logged attempt into GitHub: Project fields always; on a
+ * Mirror one logged attempt into GitHub: the Project's Target Date always; on a
  * first-ever pass also close the problem's sub-issue (comment first — repo
  * convention). Runs inside ctx.waitUntil; failures are warnings.
  */
 async function mirrorOutcome(env: Env, outcome: LogOutcome, source: string): Promise<void> {
   if (outcome.error || outcome.duplicate) return;
   const gh = github(env.GH_PAT, env.REPO);
-  await mirrorFields(projectMirror(gh, env.REPO.split("/")[0]!), outcome);
+  const mirror = projectMirror(gh, env.REPO.split("/")[0]!);
+  await mirror.setTargetDate(outcome.issue, outcome.next_review);
   if (outcome.first && outcome.result === "pass") {
     try {
       await gh.closeIssue(
@@ -248,7 +242,7 @@ async function handleSolved(request: Request, env: Env, date: string): Promise<R
       logged.push(outcomeLine(outcome));
       // One mirror for the batch: field IDs resolve once, not per problem.
       mirror ??= projectMirror(github(env.GH_PAT, env.REPO), env.REPO.split("/")[0]!);
-      await mirrorFields(mirror, outcome);
+      await mirror.setTargetDate(outcome.issue, outcome.next_review);
     }
   }
   return Response.json({ date, logged, skipped, warnings: mirror?.warnings ?? [] });

apps/api/src/mirror.ts

@@ -4,30 +4,26 @@
  * console warning and never blocks a state write. Only problem issues are
  * ever passed in, so topic rows' Target Date is never written.
  *
- * Field/option IDs are resolved by NAME per invocation (a Worker isolate is
- * short-lived; the two extra queries per mirror burst are irrelevant at this
- * volume and survive field re-creation).
+ * Target Date is the ONLY mirrored column. SRS stage and first-attempt result
+ * live exclusively in D1 — they are read from there (charts, digest, /api/stats)
+ * and duplicating them into Project single-selects bought nothing but drift and
+ * two extra writes per log. Do not re-add them here.
+ *
+ * Field IDs are resolved by NAME per invocation (a Worker isolate is
+ * short-lived; the extra query per mirror burst is irrelevant at this
+ * volume and survives field re-creation).
  */
 import type { GitHub } from "./github.ts";
 
 const PROJECT_TITLE = "Interview Prep";
 
-interface SelectField {
-  id: string;
-  options: Record<string, string>;
-}
-
 interface ProjectInfo {
   id: string;
   targetDate: string;
-  srsStage: SelectField;
-  firstAttempt: SelectField;
 }
 
 export interface Mirror {
   setTargetDate(issue: number, date: string | null): Promise<void>;
-  setStage(issue: number, stage: string): Promise<void>;
-  setFirstAttempt(issue: number, result: string): Promise<void>;
   warnings: string[];
 }
 
@@ -47,7 +43,6 @@ export function projectMirror(gh: GitHub, owner: string): Mirror {
               fields(first: 30) {
                 nodes {
                   ... on ProjectV2FieldCommon { id name dataType }
-                  ... on ProjectV2SingleSelectField { id name options { id name } }
                 }
               }
             }
@@ -62,7 +57,7 @@ export function projectMirror(gh: GitHub, owner: string): Mirror {
             id: string;
             title: string;
             fields: {
-              nodes: { id: string; name: string; options?: { id: string; name: string }[] }[];
+              nodes: { id: string; name: string }[];
             };
           }[];
         };
@@ -70,19 +65,9 @@ export function projectMirror(gh: GitHub, owner: string): Mirror {
     };
     const node = data.user.projectsV2.nodes.find((n) => n.title === PROJECT_TITLE);
     if (!node) throw new Error(`project "${PROJECT_TITLE}" not found for @${owner}`);
-    const select = (name: string): SelectField => {
-      const f = node.fields.nodes.find((n) => n.name === name);
-      if (!f?.options) throw new Error(`single-select field "${name}" missing`);
-      return { id: f.id, options: Object.fromEntries(f.options.map((o) => [o.name, o.id])) };
-    };
     const date = node.fields.nodes.find((n) => n.name === "Target Date");
     if (!date) throw new Error(`date field "Target Date" missing`);
-    info = {
-      id: node.id,
-      targetDate: date.id,
-      srsStage: select("SRS Stage"),
-      firstAttempt: select("First Attempt"),
-    };
+    info = { id: node.id, targetDate: date.id };
     return info;
   }
 
@@ -154,19 +139,5 @@ export function projectMirror(gh: GitHub, owner: string): Mirror {
           await setField(issue, project.targetDate, { date });
         }
       }),
-    setStage: (issue, stage) =>
-      attempt(`SRS Stage #${issue}`, async () => {
-        const project = await resolve();
-        const option = project.srsStage.options[stage];
-        if (!option) throw new Error(`no option "${stage}"`);
-        await setField(issue, project.srsStage.id, { singleSelectOptionId: option });
-      }),
-    setFirstAttempt: (issue, result) =>
-      attempt(`First Attempt #${issue}`, async () => {
-        const project = await resolve();
-        const option = project.firstAttempt.options[result];
-        if (!option) throw new Error(`no option "${result}"`);
-        await setField(issue, project.firstAttempt.id, { singleSelectOptionId: option });
-      }),
   };
 }

apps/cli/github.ts

@@ -1,6 +1,6 @@
 /**
- * GitHub REST plumbing shared by the issue reconcilers (close-solved,
- * close-topics).
+ * GitHub REST + GraphQL plumbing shared by the issue reconcilers (close-solved,
+ * close-topics) and the Project field reconciler (sync-project-fields).
  *
  * Importing this module is side-effect free — nothing resolves credentials or
  * touches the network until github() is awaited — so a script can import it
@@ -22,6 +22,8 @@ export interface GitHub {
   repo: string;
   /** One authenticated request against api.github.com; throws on non-2xx. */
   api(path: string, init?: RequestInit): Promise<unknown>;
+  /** One GraphQL call (Projects v2 lives here); throws on transport or query errors. */
+  graphql(query: string, variables?: Record<string, unknown>): Promise<unknown>;
   /** Every page of a list endpoint, flattened into one stream of elements. */
   list(path: string): AsyncGenerator<unknown, void, void>;
   /** Comment on an issue, then close it as completed. */
@@ -74,6 +76,24 @@ export async function github(): Promise<GitHub> {
     }
   }
 
+  async function graphql(
+    query: string,
+    variables: Record<string, unknown> = {},
+  ): Promise<unknown> {
+    const res = await fetch("https://api.github.com/graphql", {
+      method: "POST",
+      headers: {
+        authorization: `Bearer ${token}`,
+        "content-type": "application/json",
+      },
+      body: JSON.stringify({ query, variables }),
+    });
+    if (!res.ok) throw new Error(`graphql -> ${res.status} ${await res.text()}`);
+    const payload = (await res.json()) as { data?: unknown; errors?: { message: string }[] };
+    if (payload.errors?.length) throw new Error(payload.errors.map((e) => e.message).join("; "));
+    return payload.data;
+  }
+
   /**
    * Comment before closing: if the PATCH fails, the issue still carries a
    * visible note of what the automation decided, instead of failing silently.
@@ -89,5 +109,5 @@ export async function github(): Promise<GitHub> {
     });
   }
 
-  return { repo, api, list, closeIssue };
+  return { repo, api, graphql, list, closeIssue };
 }

apps/cli/sync-project-fields.ts

@@ -0,0 +1,252 @@
+#!/usr/bin/env bun
+/**
+ * Reconcile the "Interview Prep" Project's Set and Difficulty fields from the
+ * labels on each problem issue.
+ *
+ * The issue labels are the truth: `set:core|optional|deferred` and
+ * `diff:easy|medium|hard` are fixed when a problem issue is filed, and the
+ * Project's single-selects are a projection of them so the board can group and
+ * filter. This recomputes every desired value from scratch and writes only the
+ * ones that differ, so re-runs are no-ops and a backfill needs no special
+ * casing — same reconcile-don't-react shape as close-solved / close-topics.
+ *
+ * Scope guard: only items whose issue carries the `problem` label are touched.
+ * Topic, cadence and review issues have no set or difficulty, so they are
+ * skipped outright — never written, never cleared.
+ *
+ * Deliberately NOT mirrored: SRS stage and first-attempt result. Those live
+ * only in D1 (apps/api owns them; the charts, digest and /api/stats read them
+ * from there). The Project mirror writes Target Date and nothing else.
+ *
+ * Auth: needs a token carrying the `project` scope — neither the default
+ * `gh auth token` nor Actions' GITHUB_TOKEN has it. Pass one in GH_TOKEN: the
+ * PROJECT_PAT secret in CI, or the Worker's GH_PAT locally:
+ *   GH_TOKEN=$GH_PAT bun run sync-project-fields -- --dry-run
+ */
+import { github, type GitHub } from "./github.ts";
+import { markdownTable, printTable, writeStepSummary } from "./report.ts";
+
+const PROJECT_TITLE = "Interview Prep";
+
+/** Label → single-select option name, per field. Static tables, so Record. */
+const FIELDS: Record<string, Record<string, string>> = {
+  Set: { "set:core": "Core", "set:optional": "Optional", "set:deferred": "Deferred" },
+  Difficulty: { "diff:easy": "Easy", "diff:medium": "Medium", "diff:hard": "Hard" },
+};
+
+const DRY =
+  process.argv.includes("--dry-run") ||
+  process.env.DRY_RUN === "1" ||
+  process.env.DRY_RUN === "true";
+
+// ── project + field resolution ───────────────────────────────────
+
+interface SelectField {
+  id: string;
+  /** option name → option id */
+  options: Record<string, string>;
+}
+
+interface Project {
+  id: string;
+  fields: Record<string, SelectField>;
+}
+
+/**
+ * Field and option IDs are resolved by NAME, so re-creating a field in the UI
+ * does not strand this script on a dead id.
+ */
+async function readProject(gh: GitHub, owner: string): Promise<Project> {
+  const data = (await gh.graphql(
+    `query($owner: String!, $title: String!) {
+      user(login: $owner) {
+        projectsV2(first: 10, query: $title) {
+          nodes {
+            id title
+            fields(first: 30) {
+              nodes {
+                ... on ProjectV2SingleSelectField { id name options { id name } }
+              }
+            }
+          }
+        }
+      }
+    }`,
+    { owner, title: PROJECT_TITLE },
+  )) as {
+    user: {
+      projectsV2: {
+        nodes: {
+          id: string;
+          title: string;
+          fields: { nodes: ({ id: string; name: string; options: { id: string; name: string }[] } | Record<string, never>)[] };
+        }[];
+      };
+    };
+  };
+
+  const node = data.user.projectsV2.nodes.find((n) => n.title === PROJECT_TITLE);
+  if (!node) throw new Error(`project "${PROJECT_TITLE}" not found for @${owner}`);
+
+  const fields: Record<string, SelectField> = {};
+  for (const name of Object.keys(FIELDS)) {
+    const field = node.fields.nodes.find((f) => "name" in f && f.name === name);
+    if (!field || !("options" in field)) {
+      throw new Error(`single-select field "${name}" missing from "${PROJECT_TITLE}"`);
+    }
+    fields[name] = {
+      id: field.id,
+      options: Object.fromEntries(field.options.map((o) => [o.name, o.id])),
+    };
+  }
+  return { id: node.id, fields };
+}
+
+// ── items ────────────────────────────────────────────────────────
+
+interface Item {
+  id: string;
+  issue: number;
+  title: string;
+  labels: string[];
+  /** field name → current option name */
+  current: Record<string, string>;
+}
+
+async function* readItems(gh: GitHub, project: string): AsyncGenerator<Item, void, void> {
+  let cursor: string | null = null;
+  for (;;) {
+    const data = (await gh.graphql(
+      `query($id: ID!, $after: String) {
+        node(id: $id) {
+          ... on ProjectV2 {
+            items(first: 100, after: $after) {
+              pageInfo { hasNextPage endCursor }
+              nodes {
+                id
+                fieldValues(first: 25) {
+                  nodes {
+                    ... on ProjectV2ItemFieldSingleSelectValue {
+                      name field { ... on ProjectV2FieldCommon { name } }
+                    }
+                  }
+                }
+                content {
+                  ... on Issue { number title labels(first: 25) { nodes { name } } }
+                }
+              }
+            }
+          }
+        }
+      }`,
+      { id: project, after: cursor },
+    )) as {
+      node: {
+        items: {
+          pageInfo: { hasNextPage: boolean; endCursor: string };
+          nodes: {
+            id: string;
+            fieldValues: { nodes: ({ name: string; field: { name: string } } | Record<string, never>)[] };
+            content: { number: number; title: string; labels: { nodes: { name: string }[] } } | Record<string, never>;
+          }[];
+        };
+      };
+    };
+
+    for (const node of data.node.items.nodes) {
+      const content = node.content;
+      // Draft items and PRs have no issue number; nothing to reconcile.
+      if (!("number" in content) || typeof content.number !== "number") continue;
+      const current: Record<string, string> = {};
+      for (const value of node.fieldValues.nodes) {
+        if ("name" in value && value.field?.name) current[value.field.name] = value.name;
+      }
+      yield {
+        id: node.id,
+        issue: content.number,
+        title: content.title,
+        labels: content.labels.nodes.map((l) => l.name),
+        current,
+      };
+    }
+
+    const page = data.node.items.pageInfo;
+    if (!page.hasNextPage) return;
+    cursor = page.endCursor;
+  }
+}
+
+// ── reconcile ────────────────────────────────────────────────────
+
+interface Write {
+  item: string;
+  issue: number;
+  title: string;
+  field: string;
+  from: string;
+  to: string;
+}
+
+async function setField(gh: GitHub, project: string, w: Write, option: string): Promise<void> {
+  await gh.graphql(
+    `mutation($project: ID!, $item: ID!, $field: ID!, $option: String!) {
+      updateProjectV2ItemFieldValue(
+        input: {
+          projectId: $project, itemId: $item, fieldId: $field
+          value: { singleSelectOptionId: $option }
+        }
+      ) { projectV2Item { id } }
+    }`,
+    { project, item: w.item, field: w.field, option },
+  );
+}
+
+const gh = await github();
+const owner = gh.repo.split("/")[0]!;
+const project = await readProject(gh, owner);
+
+const writes: Write[] = [];
+let problems = 0;
+let skipped = 0;
+
+for await (const item of readItems(gh, project.id)) {
+  if (!item.labels.includes("problem")) {
+    skipped++;
+    continue;
+  }
+  problems++;
+  for (const [field, mapping] of Object.entries(FIELDS)) {
+    const want = item.labels.map((l) => mapping[l]).find(Boolean);
+    if (!want) {
+      console.warn(`#${item.issue} has no ${field.toLowerCase()} label — left untouched`);
+      continue;
+    }
+    const from = item.current[field];
+    if (from === want) continue;
+    if (!project.fields[field]!.options[want]) {
+      throw new Error(`field "${field}" has no option "${want}"`);
+    }
+    writes.push({
+      item: item.id,
+      issue: item.issue,
+      title: item.title,
+      field: project.fields[field]!.id,
+      from: from ?? "(empty)",
+      to: want,
+    });
+  }
+}
+
+const rows: string[][] = [["ISSUE", "PROBLEM", "FIELD", "FROM", "TO"]];
+for (const w of writes) {
+  const name = Object.entries(project.fields).find(([, f]) => f.id === w.field)![0];
+  rows.push([`#${w.issue}`, w.title.slice(0, 46), name, w.from, w.to]);
+  if (DRY) continue;
+  await setField(gh, project.id, w, project.fields[name]!.options[w.to]!);
+}
+
+if (writes.length) printTable(rows);
+console.log(
+  `\n${DRY ? "would update" : "updated"} ${writes.length} field value(s) across ${problems} problem item(s); ${skipped} non-problem item(s) skipped`,
+);
+if (writes.length) await writeStepSummary(markdownTable(rows));

apps/docs/content/(array)/739-daily-temperatures.mdx

@@ -0,0 +1,41 @@
+---
+title: '739. Daily Temperatures'
+description: Given an array of integers temperatures represents the daily temperatures, return an array answer such that answer[i] is the number of days you have to wait after the i^th day to get a warmer temperature. If there is no future day for which this is possible, keep answer[i] == 0 instead
+sidebar:
+  label: 'Daily Temperatures'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Monotonic Stack</Badge>
+
+### Example 1:
+- Input: `temperatures = [73,74,75,71,69,72,76,73]`
+- Output: `[1,1,4,2,1,1,0,0]`
+
+### Example 2:
+- Input: `temperatures = [30,40,50,60]`
+- Output: `[1,1,1,0]`
+
+### Example 3:
+- Input: `temperatures = [30,60,90]`
+- Output: `[1,1,0]`
+
+### Constraints:
+
+- `1 <= temperatures.length <= 10^5`
+- `30 <= temperatures[i] <= 100`
+
+## Solution
+
+```py
+class Solution:
+    def dailyTemperatures(self, temperatures: List[int]) -> List[int]:
+        ans = [0] * len(temperatures)
+        stack = []
+        for i, temp in enumerate(temperatures):
+            while stack and temperatures[stack[-1]] < temp:
+                prev = stack.pop()
+                ans[prev] = i - prev
+            stack.append(i)
+        return ans
+```

apps/docs/content/(array)/853-car-fleet.mdx

@@ -0,0 +1,47 @@
+---
+title: '853. Car Fleet'
+description: There are n cars at given miles away from the starting mile 0, traveling to reach the mile target
+sidebar:
+  label: 'Car Fleet'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Monotonic Stack</Badge>
+
+### Example 1:
+- Input: `target = 12, position = [10,8,0,5,3], speed = [2,4,1,1,3]`
+- Output: `3`
+- Explanation: • The cars starting at `10 (speed 2`) and `8 (speed 4`) become a fleet, meeting each other at `12`. The fleet forms at `target`. • The car starting at `0 (speed 1`) does not catch up to any other car, so it is a fleet by itself. • The cars starting at `5 (speed 1`) and `3 (speed 3`) become a fleet, meeting each other at `6`. The fleet moves at `speed 1` until it reaches `target`.
+
+### Example 2:
+- Input: `target = 10, position = [3], speed = [3]`
+- Output: `1`
+- Explanation: There is only one car, hence there is only one fleet.
+
+### Example 3:
+- Input: `target = 100, position = [0,2,4], speed = [4,2,1]`
+- Output: `1`
+- Explanation: • The cars starting at `0 (speed 4`) and `2 (speed 2`) become a fleet, meeting each other at `4`. The car starting at `4 (speed 1`) travels to `5`. • Then, the fleet at `4 (speed 2`) and the car at `position 5 (speed 1`) become one fleet, meeting each other at `6`. The fleet moves at `speed 1` until it reaches `target`.
+
+### Constraints:
+
+- `n == position.length == speed.length`
+- `1 <= n <= 10^5`
+- `0 < target <= 10^6`
+- `0 <= position[i] < target`
+- All the values of `position` are unique.
+- `0 < speed[i] <= 10^6`
+
+## Solution
+
+```py
+class Solution:
+    def carFleet(self, target: int, position: List[int], speed: List[int]) -> int:
+        stack = []
+        for pos, spd in sorted(zip(position, speed), reverse=True):
+            time = (target - pos) / spd
+            if not stack or time > stack[-1]:
+                stack.append(time)
+
+        return len(stack)
+```

package.json

@@ -9,6 +9,7 @@
     "doctor": "cd apps/docs && bun run doctor",
     "close-solved": "bun apps/cli/close-solved.ts",
     "close-topics": "bun apps/cli/close-topics.ts",
+    "sync-project-fields": "bun apps/cli/sync-project-fields.ts",
     "api:dev": "cd apps/api && bun run dev",
     "api:deploy": "cd apps/api && bun run deploy",
     "api:test": "cd apps/api && bun test src",

work/Medium/Array/739.daily-temperatures.py

@@ -0,0 +1,50 @@
+"""
+739. Daily Temperatures
+Difficulty: Medium
+https://leetcode.com/problems/daily-temperatures/
+
+──────────────────────────────────────────────────
+
+Given an array of integers temperatures represents the daily
+temperatures, return an array answer such that answer[i] is the number
+of days you have to wait after the i^th day to get a warmer
+temperature. If there is no future day for which this is possible,
+keep answer[i] == 0 instead.
+
+
+
+Example 1:
+
+Input: temperatures = [73,74,75,71,69,72,76,73]
+Output: [1,1,4,2,1,1,0,0]
+
+Example 2:
+
+Input: temperatures = [30,40,50,60]
+Output: [1,1,1,0]
+
+Example 3:
+
+Input: temperatures = [30,60,90]
+Output: [1,1,0]
+
+
+
+Constraints:
+
+        • 1 <= temperatures.length <= 10^5
+
+        • 30 <= temperatures[i] <= 100
+"""
+
+
+class Solution:
+    def dailyTemperatures(self, temperatures: List[int]) -> List[int]:
+        ans = [0] * len(temperatures)
+        stack = []
+        for i, temp in enumerate(temperatures):
+            while stack and temperatures[stack[-1]] < temp:
+                prev = stack.pop()
+                ans[prev] = i - prev
+            stack.append(i)
+        return ans

work/Medium/Array/853.car-fleet.py

@@ -0,0 +1,98 @@
+"""
+853. Car Fleet
+Difficulty: Medium
+https://leetcode.com/problems/car-fleet/
+
+──────────────────────────────────────────────────
+
+There are n cars at given miles away from the starting mile 0,
+traveling to reach the mile target.
+
+You are given two integer arrays position and speed, both of length
+n, where position[i] is the starting mile of the i^th car and speed[i]
+is the speed of the i^th car in miles per hour.
+
+A car cannot pass another car, but it can catch up and then travel
+next to it at the speed of the slower car.
+
+A car fleet is a single car or a group of cars driving next to each
+other. The speed of the car fleet is the minimum speed of any car in
+the fleet.
+
+If a car catches up to a car fleet at the mile target, it will still
+be considered as part of the car fleet.
+
+Return the number of car fleets that will arrive at the destination.
+
+
+
+Example 1:
+
+Input: target = 12, position = [10,8,0,5,3], speed = [2,4,1,1,3]
+
+Output: 3
+
+Explanation:
+
+• The cars starting at 10 (speed 2) and 8 (speed 4) become a fleet,
+meeting each other at 12. The fleet forms at target.
+
+• The car starting at 0 (speed 1) does not catch up to any other
+car, so it is a fleet by itself.
+
+• The cars starting at 5 (speed 1) and 3 (speed 3) become a fleet,
+meeting each other at 6. The fleet moves at speed 1 until it reaches
+target.
+
+Example 2:
+
+Input: target = 10, position = [3], speed = [3]
+
+Output: 1
+
+Explanation:
+
+There is only one car, hence there is only one fleet.
+
+Example 3:
+
+Input: target = 100, position = [0,2,4], speed = [4,2,1]
+
+Output: 1
+
+Explanation:
+
+• The cars starting at 0 (speed 4) and 2 (speed 2) become a fleet,
+meeting each other at 4. The car starting at 4 (speed 1) travels to 5.
+
+• Then, the fleet at 4 (speed 2) and the car at position 5 (speed 1)
+become one fleet, meeting each other at 6. The fleet moves at speed 1
+until it reaches target.
+
+
+
+Constraints:
+
+        • n == position.length == speed.length
+
+        • 1 <= n <= 10^5
+
+        • 0 < target <= 10^6
+
+        • 0 <= position[i] < target
+
+        • All the values of position are unique.
+
+        • 0 < speed[i] <= 10^6
+"""
+
+
+class Solution:
+    def carFleet(self, target: int, position: List[int], speed: List[int]) -> int:
+        stack = []
+        for pos, spd in sorted(zip(position, speed), reverse=True):
+            time = (target - pos) / spd
+            if not stack or time > stack[-1]:
+                stack.append(time)
+
+        return len(stack)