leetcode/.claude/skills/problem-approach/SKILL.md

name, description

name	description
problem-approach	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):

   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:

   - `-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:

   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:

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):

## 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.