Codility

Codility is primarily a technical hiring platform used by companies like Airbnb, Duolingo, and Booking.com, but it also functions as a serious coding practice environment with 1,200+ validated problems across 80+ programming languages and frameworks. Problems test real coding skills, not just algorithms, with a focus on code quality, efficiency, and writing clean solutions. The difficulty and rigor match what companies expect in technical interviews.

Use this if you're preparing for tech interviews at serious companies, want to benchmark your coding skills against industry standards, or need a challenging problem set beyond basics. Expect real-world difficulty and honest feedback on your solutions; this isn't a gentle learning platform, it's a training ground for the interview gauntlet.