Every judge from the old flat list, tiered by how much training value it actually has. Pick from the top tiers first; the rest are here for completeness, not because they’re all worth your time. For curated purpose-built sources (not raw judges), see Resources.
S+ — the two that matter most
| Judge | Why |
|---|---|
| Codeforces | Largest rated archive, frequent rounds, the deepest editorial culture; the default platform |
| AtCoder | Clean problem statements, excellent editorials, ABC/ARC/AGC ladder, strong for rating-matched practice |
S — ICPC-grade practice
| Judge | Why |
|---|---|
| Codeforces Gyms | Past ICPC regionals and mirrored contests, closest thing to real ICPC practice |
| Library Checker | Verifies raw algorithm implementations against strict time limits — the standard place to test a library |
| CSES Problem Set | One problem per core topic, exactly the right difficulty to build foundational fluency |
| ICPC Past Problems | Official World Finals archive |
A+ — major national/community judges
| Judge | Why |
|---|---|
| Luogu | Largest Chinese CP community; huge problem set with discussion and 题解 (solution write-ups) per problem |
| Baekjoon (BOJ) | Korean judge, 10000+ problems, solved.ac layers a rating system on top |
| Kattis / Open Kattis | Used by many university ICPC programs directly as a judge |
| OJ.uz | The place for past IOI/olympiad problems specifically |
| SPOJ | Old but huge classic archive, many unique problems not found elsewhere |
A — solid general-purpose archives
| Judge | Why |
|---|---|
| CodeChef | Long challenges and cook-offs, good for slower, harder problems |
| USACO | The US olympiad’s own judge; bronze→platinum problems are the best free ladder below IOI difficulty, see IOI Resources |
| HackerEarth | Monthly contests plus a large practice archive, widely used for hiring challenges too |
| DMOJ | Canadian judge, strong olympiad-style problem set |
| TopCoder | SRMs (historic archive) and marathon matches |
| HackerRank / LeetCode | Interview-leaning, still fine for implementation practice |
| TopH | Bangladeshi judge with regular contests |
| LightOJ | Beginner-to-intermediate problem archive |
B — niche but legitimate
| Judge | Why |
|---|---|
| CS Academy | Interactive problems, built-in graph editor |
| Codewars | Rank-based coding kata in many languages |
| GeeksforGeeks | CS-theory-adjacent practice problems |
| HiHoCoder | Chinese/English practice and recruitment challenges |
| Infoarena | Romanian, 1500+ algorithmic problems |
| Nowcoder | Chinese, contests plus a large archive |
| UOJ | Chinese, advanced problem set |
| Libre OJ (LOJ) | Chinese, competitive problems — not to be confused with the Bangladeshi LightOJ above |
| Hydro | Chinese, modern multi-language judge |
| Eolymp | Large olympiad problem collection, hosts several national/regional camps and contests |
| Yukicoder | Japanese |
| Aizu Online Judge (AOJ) | Japanese university judge |
| Beecrowd | Brazilian, formerly URI |
| Advent of Code | Yearly, lighter puzzle-style problems |
| Project Euler | Math-heavy, not CP-style but good number-theory practice |
Legacy — classic archives, still used for old problems
| Judge | Note |
|---|---|
| UVa Online Judge | The original, referenced by every early CP book |
| POJ | Peking University, classic problem phrasing |
| ACM HDU | Hangzhou Dianzi, huge classic Chinese archive |
| ACM Timus | Ural, rigorous classic problem set |
| onlinejudge.org | Umbrella judge hosting the UVa archive |
National judges, by region
| Region | Judges |
|---|---|
| Russia / CIS | ACMP, ACM Ural, NEERC Archive, Yandex Contest, Inf‑Open, SnarkNews |
| Belarus | ACM BSU, ACM BSUIR, DL GSU, Solve.by |
| China | see A+/B above (Luogu, UOJ, Nowcoder, Hydro, Libre OJ); also BZOJ / Dark BZOJ mirror |
| Japan | Aizu, Yukicoder |
| Korea | Baekjoon, rated via solved.ac |
| Uzbekistan | RoboContest, KEP |
| Poland | Potyczki Algorytmiczne |
| Croatia | COCI |
| Romania | Infoarena, Kilonova |
| Indonesia | TLX |
| Brazil | Beecrowd |
| Caribbean | COJ |
| Ukraine | AlgoTester — hosts national olympiad stages and the Algorithmic Leo Camp |
Contest systems and platforms (not standalone archives)
- Polygon — problem-setting platform used to build Codeforces-style contests.
- clist.by — aggregates upcoming contests across every judge above; the single best “what’s on this week” page.
- Virtual Judge (vjudge) — runs virtual contests pulling problems live from other judges.
- ITMO School Contests / ITMO Trains — training-contest infrastructure around NEERC.
- Innopolis PCMS — contest management system, used for several Russian regional contests.
Clusters and training camps
- Petrozavodsk Camp — the highest-level recurring ICPC training camp; problems are notoriously hard and well-set.
- Universal Cup — global team contest series explicitly modeled on Petrozavodsk-style difficulty.
- Open Cup — long-running Russian team contest series, open to all, feeds into Petrozavodsk-style training.
- IPSC — Internet Problem Solving Contest, a yearly, deliberately varied-difficulty team event.