Spoj - Sphere Online Judge created for the coders' community to train, to learn and to make progress.
LeetCode - Practice and level up your development skills and prepare for technical interviews.
Codewars - Achieve code mastery through challenge.
Project Euler - Project Euler is a series of challenging mathematical/computer programming problems that will...
Exercism - Download and solve practice problems in over 30 different languages.
HackerRank - HackerRank is a platform that allows companies to conduct interviews remotely to hire developers and for technical assessment purposes.