I, like many others, am doomed to the 500 error when trying to sign up to Codewars (github.com/Codewars/codewars.com/i...). It seems like a pretty good platform apart from this long standing issue.
Sorry to hear that, I haven't used it in a while. True their quality of service and katas is not the greatest but the "battle" feeling is great.
For more CS and Math problems I recommend leetcode.com, and for interviews interviewbit.com
Other more Gamified alternatives are codecombat.com/ and codingame.com/, also good for non-tech people that want to switch to a dev-related position and need something fun to do while learning. These two I recommend after scratch.mit.edu/
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