Build it yourself.

More interviews now include an “implement this component” round: a rate limiter, an LRU cache, a consistent hashing ring. Each lab here is one of those, taken straight from a lesson. Write Python, JavaScript or TypeScript in the browser and run it against example tests and hidden edge cases.

• No install, no serverPython runs in your browser (Pyodide, CPython compiled to WebAssembly; the first run downloads it once, ~6.5 MB). JavaScript and TypeScript run in a sandboxed worker.
• Practice like the real roundInterview mode puts a clock on it, takes the hints away and asks the follow-ups. Chaos and scale tests show what passing tests miss.
• Works offlineSave a lab (or a whole world) and it runs with no internet. Your code autosaves in this browser.