UVa

The Valladolid University Online Judge (UVa) Problemset
https://onlinejudge.org/

History
This site has been around for quite awhile, with over 23 million submissions as of December 12th, 2019.

Problems
Please use the Problem Template when adding new problems; see Help:Editing for a step-by-step guide.

Tools

 * uHunt is a tool for UVa online-judge that keeps statistics, provide selections of problems to solve, and exposes a web API for other web developers to build upon it.
 * uDebug We are an enthusiastic community of competitive programmers who help each other out by answering questions on chat, providing hints and solutions to problems from several online judges, furnishing test input and sharing feedback. We do all this while having fun and fostering a sense of camaraderie!