[touchysubjects]

Codingbat looks cool, hadn't heard of it before. You might also try Project Euler, another programming/math-problem solving site where "the problems range in difficulty and for many the experience is inductive chain learning. That is, by solving one problem it will expose you to a new concept that allows you to undertake a previously inaccessible problem. So the determined participant will slowly but surely work his/her way through every problem."

https://projecteuler.net/