LeetCode

How this question breaks down

1. 1JudgingCore25 minAccept a program, queue it, run it against every test on an isolated worker, and get a trustworthy verdict back to the coder in seconds, even when workers crash mid-run.
2. 2SandboxingCore25 minRun a stranger's program so it cannot escape, reach the network, see the answers or starve its neighbours, and time it so the same program gets the same verdict every time.
3. 3ContestsCore22 minOpen four problems to 40,000 people at the same second, judge their burst of submissions fairly, and keep a live ranking that every contestant can trust and a hundred thousand spectators can refresh.

Work through the parts in order and you’ve covered the full design. Each part is one topic.