I'm shocked Codility isn't on here yet. https://codility.com/programmers/lessons/1-iterations/ https://codility.com/programmers/lessons/1-iterations/ They're r

I'm shocked Codility isn't on here yet. https://codility.com/programmers/lessons/1-iterations/ https://codility.com/programmers/lessons/1-iterations/ They're really strong on language-agnostic testing with emphasis on CS concepts. That's great and typical, but they have the study material linked directly to each lesson. I've had great interview experiences through their platform. (shameless plug warning) I liked it enough that when I went through Flatiron School, I made a Ruby gem to see their lessons in the command line. https://rubygems.org/gems/rubedility https://rubygems.org/gems/rubedility