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An online judge is an online system to test programs in programming contests. They are also used to practice for such contests. Many of these systems organize their own contests.
The system can compile and execute code, and test them with pre-constructed data. Submitted code may be run with restrictions, including time limit, memory limit, security restriction and so on. The output of the code will be captured by the system, and compared with the standard output. The system will then return the result. When mistakes were found in a standard output, rejudgement using the same method must be made.
Online Judges have ranklists showing users with the biggest number of accepted solutions and shortest execution time for a particular problem.
- ACM International Collegiate Programming Contest
- International Olympiad in Informatics
- Sphere Online Judge
- Valladolid Problem Archive
- Kattis Online Judge
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- Light Online Judge , over 350 problems, C/ C++/ Pascal/ Python/ Java supported. It has its own chat room, forum, online compiler, virtual contests, tool to compare statistics, social bookmarking like adding friends. Any user can arrange contests even with his own problems after solving at least 25 problems. Problems are categorized.
- Codeforces, more than 15 language supported. This site regularly arranges contests and has a rating system.
- Arbiter, online judge at Sharif University in Iran (currently supporting English and Persian). Use its public environment 'ACM_Training' to access the main problem set.
- ShareCode ACM Online Judge , The Online Judge and Social Network for ACMers.
- URI Online Judge, over 500 problems (for beginners and advanced users), supports C++ and Java (available in English and Portuguese).
- UVa Online Judge , over 5000 problems, C/ C++/ Pascal/ Java supported.
- .Campion , over 900 problems, C/ C++/ Pascal supported. Only Romanian language.
- Caribbean Online Judge , C/ C++/ Pascal/ Java/ Python/ Perl/ Ruby/ PHP/ C# supported. English and Spanish.
- Fuzhou University Online Judge, over 800 problems. Popular among the Chinese.
- Harbin Institute of Technology Online Judge, over 1800 problems. Popular among Chinese people.
- Lviv National University Online Judge, over 100 hard ACM-type problems. Only Ukrainian language.
- Moscow Problem Archive with Online Judge, supports C/C++/Java/Free Pascal/Perl/Python/Haskell/Kylix/Lua/OCaml/Ruby/Scheme.
- Peking University Online Judge, over 2800 problems including more than 200 original problems, supports C/C++/Pascal/Java/Fortran.
- Beijing Normal University Online Judge, more than 500 original problems with virtual judge functions, supports C/C++/Pascal/Java/Python.
- University of Science and Technology of China Online Judge, over 300 problems, supports C/C++/Pascal/Java.
- Saratov State University Problem Archive with Online Judge, over 300 problems
- Tianjin University Online Judge, over 2000 problems, supports C/C++/Pascal/Java, virtual contests supported.
- Teddy Online Judge, includes problems in Spanish, supports C/C++/Python/Java/Perl/PHP.
- USACO|USA Computing Olympiad Training Site, hundreds of problems with complementary tutorials for incrementally expanding competitive programming skills; supports C, C++, Pascal and Java programming languages; targeted to pre-college students.
- Valladolid Problem Archive with Online Judge, about 2500 problems, supports C, C++, Pascal and Java programming languages
- Zhejiang University Online Judge, over 2400 problems, supports C, C++, Pascal and Java.
- Timus Online Judge, The Timus Online Judge is the largest Russian archive of problems from different programming contests. Accepts solutions in C/C++, Java, Pascal, C#, Go, Haskell and Scala.
- z-training , over 900 problems, C/ C++/ Pascal/ Java/ Python and 8 other languages supported. Provides PHP APIs for building custom online judges.
- Shenyang Institute of Engineering Online Judge, The SIE Online Judge contains over 200 problems for beginners, Chinese and English.
- Open Computer Science Community and Online Judge, Open CS has a permanently growing problemset, that users can also contribute to. It also has a collection of computer science-related articles.
- Olimps! Server for Automated Program Testing, over 500 problems, supports C, C++ and Pascal. Sorting by hardness, chronology and ID is available. Latvian language only.
- Baekjoon Online Judge, over 2000 problems, supports C, C++, Java, and 22 languages. Korean language only.
- NBUT Online Judge An Online Judge System written by ThinkPHP. Over 400 problems including English ones.
- Virtual Judge Submitter An opensource  Virtual Judge APIs for Virtual Judge developers.
Some of the online judges designed for programming courses are:
- URI Online Judge, over 600 problems (for beginners and advanced users), supports C++ and Java (available in English and Portuguese).
- URI Online Judge Academic, tool for professors to create disciplines and homework using problems available at URI Online Judge.
- Online Judge Plugin for Moodle 2.x, can automatically grade C/C++/Java/Python/Perl/C# and more 40+ programming language assignments.
- Codejudge, an online judge that supports C/C++/Java/Python, an open source project written in PHP and Java using MIT License.
- HUSTOJ, HUST Online Judge,C/C++/Pascal/Java/Ruby/Bash/Python/PHP/Perl/C# supported, an open source OJ system using GPL2.0 license, which support LiveCD mode and FPS format.
- Sharif Judge, a free and open source online judge for programming courses, developed at Sharif University of Technology, supports C, C++, Java and Python
- FPS, Free Problem Set, an open source problemset exchange format based on XML, which providing more than 400 free problems in FPS format.
- SIO2, an open source online judge platform written in Python
- Infoarena, over 1440 problems, C/ C++/ Pascal supported, over 20000 users. Romanian language only.
- BGCoder.com, A lot of problems and contests, C++ and C# supported.
- Codechef.com, monthly long(10 days) and short(3.5 hrs) contests, problems for practice etc., supports over 35 languages.
Open-source project for running contests: