HashMap
题目
通过index和value来进行1对1映射（这里要求我们的value range和index range基本相近，在之后通过简单的swap可以rearrange values）
- Leetcode 41. First Missing Positive
通过key来分组
- Leetcode 6. ZigZag Conversion
- Leetcode 49. Group Anagrams
记录并查询位置
- Leetcode 380. Insert Delete GetRandom O(1) (remove时记得用到swap的技巧）
- Insert Delete GetRandom O(1) - Duplicates allowed (remove时记得用到swap的技巧）
- Leetcode 311. Sparse Matrix Multiplication
记录并查询当前key最后出现的位置
- Shortest among Longest Substrings after Split
- Longest substring that contains k distinct characters where input is a char stream. (谷歌onsite)
- Leetcode 359. Logger Rate Limiter
记录并查询数量/count
- Leetcode 348. Design Tic-Tac-Toe
- Leetcode 383. Ransom Note
- Leetcode 362. Design Hit Counter
- Leetcode 387. First Unique Character in a String
- Leetcode 389. Find the Difference
- Leetcode 30. Substring with Concatenation of All Words
- Leetcode 36. Valid Sudoku
- Leetcode 391. Perfect Rectangle (思路非常巧妙的一道题，通过统计每个corner出现的次数以及大矩形的面积来判断)
求对称点
- Leetcode 356. Line Reflection
求关注者／求neighbor
- Leetcode 355. Design Twitter
LRU思想 （queue + HashMap）这样可以达到O(1)的查找以及增删
- Leetcode 379. Design Phone Directory