Competitive 10 sites
Algorithmic sport and skill sharpening.
Codeforces
Multiple rated rounds weekly. Explicit AI-use restrictions during main contests.
AtCoder
ABC/ARC/AGC/AHC lineups. Strict AI rules on ARC/AGC.
CodeChef
Core contests free. Pivoted heavily toward student placement prep.
Kaggle competitions
More active than ever in 2026. Free with GPU/TPU notebook quotas.
Project Euler
Removed leaderboard due to cheating, adding rolling rank table. Free.
Advent of Code
Scaled to 12 puzzles + no global leaderboard in 2025 due to AI solvers and DDoS. Private leaderboards still work. Free.
CSES Problem Set
Not a contest platform — the algorithm learning problem set. No ads, no card.
TopCoder
Under Wipro. Pivoting to AI challenges vs classic SRMs. Community thinner than peak years.
HackerEarth
Firmly pivoted to B2B hiring/assessment. HourStorm + monthly circuits still free.
SPOJ
Maintenance mode — no major new contests since 2022. Still a valuable practice archive.