Topological sorting

Implement prototype(n, edges) and solve(n, edges). Vertices are 0..n-1; each directed edge [u, v] requires u before v. Return the lexicographically smallest valid full Number ordering, or null if any directed cycle prevents one. Duplicate edges count once. n=0 returns [].

For n=3 and edges [[0, 2], [1, 2]], return [0, 1, 2]. Negative n or invalid endpoints throw RangeError.

Prototype: test permutations in lexicographic order. Final: indegrees plus a smallest-first legal frontier. Preserve edges on success and failure.

All numeric inputs and finite arithmetic results in this lesson are safe-integer Numbers. Use null for documented absence, not undefined or a numeric sentinel. Return actual booleans where the contract requires them. Do not add validation requirements beyond the documented inputs.

Run with Node.js 20+ from this directory:

node main.mjs --stage prototype
node main.mjs --stage final

Edit exercise.mjs; keep main.mjs unchanged. Both stages begin as unfinished learner functions. The website uses the same test functions in a separate browser worker. Tests check deterministic examples, boundaries, documented validation, input preservation, and stage-specific cases. Passing tests does not prove the required time complexity.