leetcode 30-days-of-code happy-number maximum-subarray best-time-to-buy-and-sell-stock single-number 30-day-leetcoding-challenge leetcode-30days-challenge move-zeros group-anagrams backspace-string-compare middle-of-the-linked-list diameter-of-binary-tree last-stone-weight counting-elements contiguous-array perform-string-shifts product-of-array-ecxept-self valid-parenthesis-string may-leetcoding-challenge