在职争取用6个月准备跳槽，从今天开始记录

12/4 Wed:
42, trap rain water, two pointers
136, single number, bit manipulation, time O(n), space O(1)

补充内容 (2019-12-5 10:13):
2, add two number, Linked List, T O(n1+n2), S O(1)

12/5 Thu: 146, LRU cache, hashtable + list (i.e. doubly linked list), unordered_map<int, pair<int, list<int>::iterator>> cache; list<int> used; // cache is key, pair<value, key_iterator_in_list>

12/15 Sunday: BFS: 559, 690