Practice
Problem database
273 problems across 197 proof, 44 numerical, 32 coding. Every problem has a complete worked solution, and most have a hint ladder you can climb one rung at a time. 32 come with a code editor and run in your browser against test cases — marked run in the table. The hardest sit in The Abyss.
Difficulty runs Foundation → Olympiad / Extreme. It describes the mathematical maturity a problem assumes, not how long it takes — the time estimate is separate, and both are honest estimates rather than measurements.
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