Hackerrank challenges: pretty good, a lot of them make you think a bit, or look up a mathematic formula

Hackerrank challenges using a functional language: List.fold

Try doing Codechef challenges in a functional language, I found it way more hardcore.

Eg. Exploiting laziness to implement dynamic programming or making types for a Fenwick tree or using streams for handling huge inputs (or just because, why not)

The site is extremely meh in comparison to hackerrank though.