It would be very nice if Leetcode featured a set of problems that simply tell you to implement primitive algorithms. I'm in a spot where I need to learn how to implement merge sort efficiently, and Leetcode's smooth interface and test cases are such an immense help when doing this. There are Leetcode problems where merge sort may be used as part of the solution, but (1) I would have to find said problems and (2) there might be additional logic that needs to implemented as well as merge sort. Sure, there is Hackerrank, but the cumbersome input parsing and paywalled test cases drive me insane.