Every classic here is a real, public problem (LeetCode / Codeforces number attached), mapped back to the 3D world that trains it. Three hints, first principles, complexity, a working solution — and for most, an in-browser judge: write the function, run the tests. Solved something? Every problem also has a full walkthrough in the Solve Hall. Statements are paraphrased in our own words; company mentions are folklore, not verified claims. Warm-ups first, hard classics last, no shortcuts.
How to use the arena (the honest method)
Never read the solution first. Re-derive the structure: which world is this? What does that world promise?
Open hints one at a time, only after a real attempt — hint 3 is the key idea.
Practice runners: make the tests pass, then re-solve from a blank editor the next day. Recall beats re-reading.
Only after a real attempt, read the Solve Hall walkthrough — the wall, the turn, the dry run, the edges.
Say the complexity out loud before checking. Interviewers grade the narration as much as the code.
Trained by a world? Go replay its story — the muscle memory transfers.