CodeForces

Codeforces is an online competitive programming platform and community that hosts regular programming contests and maintains a large archive of algorithmic problems. It was founded in 2009 by Mike Mironov (a.k.a. MikeMironov) and is based in Russia. The site provides problem sets, interactive judging, and a rating system that tracks user performance across contests.

Contests and rating: Codeforces runs multiple rounds, including Codeforces Rounds and Educational Rounds, often with Div 1 and Div 2 divisions. After each contest, participants gain or lose rating points according to their relative performance. Users have global handles and can participate in virtual contests or practice.

Problemset and features: The platform hosts a large problem database with tags (such as dynamic programming, graphs, number theory) and difficulty labels. Each problem includes statements, sample tests, and sometimes official editorials. In addition to contests, Codeforces hosts Gym where teams practice problems from external contests, and Blogs where users publish tutorials.

Community and impact: Codeforces has a large, active community of competitive programmers and is widely used for learning and recruitment. It is common for contestants to prepare for university programming contests or company interviews by solving problems on Codeforces and by participating in rounds that test speed and strategy.

Platform and access: The site is available at codeforces.com. Users can customize notifications, track rating histories, and use features like problem tags and stat pages.