I started playing online-judge since I was a high school student first time learning C++. At that time, UVA Online Judge was the most popular one. The system offers problem sets related to algorithm, data structure, number theory, and other fundamental knowledge about programming. It is fun and actually make your brain bigger.
Some other interesting online judge sites are:
After graduated from college, I programed more on data storage, business logic, application interface and almost have no time to play online judge. Recently, to prepare job interviews, I start playing online judge again and find it still joyful.
The community is getting bigger and advanced. For example, HackerRank has a discussion fourm and supports almost all modern languages. There are also many varieties:
- Codecademy - providing basic trainings.
- Check IO - for Python programmers.
- kaggle - for data ccientists.
Thanks to all the online judge communities, you really make the world better.