[ ] Repeated DNA Sequences, We can do one-pass O(N) algorithm. [ ] Longest Substring Without Repeating Characters, No idea. [ ] Populating Next Right Pointers in Each Node, No idea the meaning of 'constant space' [ ] Populating Next Right Pointers in Each Node II, AC, but solution is wrong since not only use constant space [ ] Count Complete Tree Nodes: Cheating, but works [ ] Find the Duplicate Number: Could use Linked List Cycle II to solve it, very tricky. [ ] H Index II: Corner case, do it again [ ] Multiply Strings: AC by luck, do it again