https://stackoverflow.com/questions/22439177/why-stringbuilder-is-much-faster-than-string
Iteratuve inorder multiple qs https://leetcode.com/problems/validate-binary-search-tree/discuss/32112/Learn-one-iterative-inorder-traversal-apply-it-to-multiple-tree-questions-(Java-Solution)
similar qs https://leetcode.com/problems/combination-sum/solution/
Union find https://leetcode.com/tag/union-find/
Topo sort https://leetcode.com/discuss/general-discussion/1078072/introduction-to-topological-sort