Codeforces
Compete in timed algorithm contests and raise your rating
Codeforces runs short online programming contests several times a week. In each round you get a set of problems, write solutions in the language of your choice and submit them to be tested. Faster solves earn more points. In many rounds you can also hack other people's solutions by finding a test that breaks them.
Free · Login needed · Browser · Intermediate
About Codeforces
It trains fast algorithm design and careful coding under time pressure. An Elo-style rating tracks your level, and rounds are split into divisions, from Div. 4 for newer players up to Div. 1 for the strongest. It suits anyone serious about competitive programming.
Taking part is free, but you need an account. Past contests stay open for practice and virtual runs, and there is a large problem archive and a blog.
On Breakpoint since 23 Oct 2026.