| 1. Two Sum | optimize if else (in or not in?), subtraction | |
| 14. Longest Common Prefix | | |
| 169. Majority Element | count and balance the appearance to at least 0, change to new number once they're back | |
| 217. Contains Duplicate | sorting, check i-1 vs i or using set | 1 |
| 242. Valid Anagram | hasmap with get, how to check character in s has the same frequency to t (different order) | |
| 303. Range Sum Query - Immutable | prefix | |
| 383. Ransom Note | | |
| 409. Longest Palindrome | | |
| 448. Find All Numbers Disappeared in an Array | negate value for existing number, positive value is the missing one | |
| 705. Design HashSet | | |
| 706. Design HashMap | | |
| 1929. Concatenation of Array | | |