Whichever one you do the most exercises on! I haven't used Leetcode before, but Hackerrank is really decent. I would suggest working through the challenges in t

Whichever one you do the most exercises on! I haven't used Leetcode before, but Hackerrank is really decent. I would suggest working through the challenges in the previous contests (https://www.hackerrank.com/contests https://www.hackerrank.com/contests) as they are more interesting and harder to solve than the practice exercises.