CodeCrafters

Choose a system you use and want to understand internally. Work in your own editor, then push code to run the challenge tests and receive feedback. The staged tasks make a large implementation more approachable. This suits learning how a protocol or system works through code rather than practising only isolated algorithm questions. Instructions and hints can be read for free. Without membership, tests and submissions cover the first two stages of each challenge and all stages of challenges currently marked Free; membership removes that stage limit.