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There are many sites like this, with different focuses:

Project Euler (mathematical problems): https://projecteuler.net/

Rosalind (bioinformatics): http://rosalind.info/

Are my two favorites. There are others, of course.

Codility has a good choice of languages; https://codility.com/train/

Interesting, I've been enjoying https://www.codeeval.com/ which also has the headhunter based model and supports a few more languages, but doesn't have the time limits like a real code interview usually would.

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