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Codeforces - Main contest platform with rating and problem archive
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Codeforces Gyms - Training contests and past ICPC regionals
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Leetcode - Interview‑focused problems and weekly contests
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SPOJ - Classic algorithmic problem archive
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TLE Elementor - Curated competitive programming sheet
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GeeksforGeeks - CS theory and coding problems
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Striver A‑Z DSA Sheet - Complete DSA roadmap
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TopH - Bangladeshi online judge with regular contests
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CSES Problem Set - Well‑known algorithm problem set
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Codewars - Rank‑based coding challenges in many languages
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HackerEarth - Practice problems and monthly challenges
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Light OJ - Beginner‑friendly problem archive
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OJ.uz - Collection of past olympiad problems
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ICPC Past Problems - Archive of past ICPC finals problems
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TopCoder - SRMs and marathon matches (platform)
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HackerRank - Skill‑based coding challenges
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Project Euler - Math‑oriented programming problems
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AtCoder - Japanese contest platform (regular contests)
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Google Coding Competition Archive - Past problems from Google contests (archive only)
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ACM Timus - Ural Federal University problem set
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ACM HDU - Hangzhou Dianzi University online judge
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Aizu Online Judge - Japanese university OJ
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POJ - Peking University problem archive
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Online Judge - Classic competitive programming judge featuring thousands of algorithmic problems
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UVa Online Judge - Classic OJ with huge problem set
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CS Academy - Interactive contests and graph editor (platform)
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CodeChef - Monthly long challenges and cook‑offs (platform)
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Libre OJ - Chinese OJ with competitive problems
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Luogu - Large Chinese CP community
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BZOJ - Advanced Chinese OJ
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Dark BZOJ - Mirror of BZOJ
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Hydro - Modern multi‑language OJ platform
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Nowcoder - Chinese OJ with contests
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DMOJ - Canadian OJ used for many contests
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Newton School - CP problems for learning
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RoboContest - Uzbek online judge
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Eolymp Basecamp - Collection of olympiad problems
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Code360 by Naukri - Interview prep and coding problems
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TLX - Indonesian OJ for competitions
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PrepBytes MyCode - Placement practice problems
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UOJ - Universal Online Judge (Chinese)
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KEP - Uzbek educational OJ
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BestCoder - HDU weekly contest platform
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Open Kattis - Contest platform used in many universities
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Cups.Online - Russian contest platform (Moscow workshops)
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ACMP - Russian OJ for school and university
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BinarySearch - Live coding interview practice
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ITMO School Contests - NEERC school contest archive
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Beecrowd - Brazilian OJ (formerly URI)
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ACM Ural - Russian Ural contests
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NEERC Archive - Northern Eurasia contest archive
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DL GSU - Belarusian OJ
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CodeDrills - ICPC‑style contest platform
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Universal Cup - Team contest series (platform - the competition itself is listed below)
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AlgoTester - Ukrainian algorithmic contests (platform)
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Yukicoder - Japanese OJ for competitive programming
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Kattis - Main Kattis problem archive
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COCI - Croatian Open Competition in Informatics (archive)
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ACM BSU - Belarusian State University contest archive
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Inf‑Open - Russian informatics open contest archive
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Petrozavodsk Camp - High‑level training camp (practice)
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SnarkNews Contests - Contest archive
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AlgoLeague - Algorithmic league platform
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Potyczki Algorytmiczne - Polish algorithmic contest platform
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AlgoGE - Algerian CP platform
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IOI Statistics - IOI problem statistics
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Yandex Contest - General Yandex contest archive
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ACM BSUIR - BSUIR open contest archive
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AZSPCS - Puzzle contest archive
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UCLA CodeSprint - UCLA ACM competition (archive)
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BSUIR Contest - BSUIR contest system
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Sort‑Me - French CP platform
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SupeCoder - Spanish/Portuguese coding contests (platform)
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Innopolis PCMS - Innopolis contest system
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Kilonova - Romanian OJ
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ITMO Trains - ITMO training contest archive
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MIPT Edu - MIPT educational contests
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Teamscode - High school contest platform
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COJ - Caribbean Online Judge
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PetrSU ACM - Petrozavodsk State University contest archive
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MITIT - MITIT contest archive
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Informatics MCCME - Moscow State informatics archive
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Calico - UC Berkeley Calico contest archive
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Contest Pizza - Fun contest platform
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Cereal Codes - Cereal Codes contest platform
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Constructor University - Constructor University contest archive
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Solved.ac - Korean rating system for BOJ problems
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Solve.by - Belarusian solve platform
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Algo Muse - Research-based algorithmic problems.
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Baekjoon Online Judge - Korean Online Judge. 10000+ problems. Supports 60+ languages.
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CodeMarshal - Real-world contests online!
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HiHoCoder - Chinese and English problem-solving practice and recruitment challenge site.
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Infoarena - Romanian Online Judge. 1500+ algorithmic problems
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LavidaOnlineJudge - Korean Online Judge(Half English). 1300+ problems.
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ShareCode.io - Online Judge and contest host with a lot of algorithmic problems in the archive to practice.
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Problems That Will Make You Learn Something New - Curated list of problems that teach new concepts
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Codeforces Topic-wise Questions Sheet - Topic‑wise CP sheet
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LeetCode Topic-wise Questions Sheet - Topic‑wise assignments from Kunal’s DSA bootcamp