| Kattis | Kattis is a server used to host many competitions, incl. NCPC (The Nordic Collegiate Programming Competition) in Denmark also labelled the Danish Championship in Programming. |
| Topcoder | TopCoder is a company that regularly organizes programming competitions in various disciplines. They organize on weekly basis algorithm competitions. Single Round Matches (SRMs) typically consist of three problems by increasing difficulty, and the coding phase is 75 minutes followed by a 15 challenge phase where you can try to get points by spotting errors in other peoples solutions. Programming is by default done in the TopCoder Arena, a Java applet supporting online compilation, testing and submitting of scores. Topcoder Open (TCO15) is a yearly event, consisting of multiple rounds with an onsite final. You need a profile to login. |
| CodeForces | Codeforces organizes community driven programming contents, that organizes weekly competitions, each competition lasting for about two hours. Compilation is done offline. You download random generated input instances and upload your code together with the output generated by your program on the input instance. Input instances are typically only valid for a few minutes. Hereafter a new input instance can be requested. You need to be a registered user to submit solutions. |
| Google Code Jam | As the name suggests, a coding competition organized by Google. Since 2008, Google organizes each year a Google Code Jam, consisting of a number of rounds with an onsite final. In 2014 over 25.000 participated in the qualificication round. Problems from the previous Jams are available on the website for practicing. |
| UVa Online Judge | A large collection of programming problems. Solutions can be uploaded and judged online. A user account is required. |
| CodeChef | Another online judge hosting many problems and contests. A user account is required for testing solutions. |
| ACM-ICPC | ACM International Collegiate Programming Contests is an international student competition, where teams consists of three contestants. Danish students can participate in the Northwestern European Regional Contest (NWERC), where the winners will advance to the ACM Programming Contest World finals. Problems from previous ACM-ICPC Regionals and World Finals can be found and tried at the ACM-ICPC Live Archive. |
| ProjectEuler | Mathematical oriented puzzles. Answers are just a fixed single number. You need to be logged in with a user account to submit and verify your answer. |
| USACO | The USA Computing Olympiad Training Program Gateway is a website for beginners. |