record troubles here...
5.2:
Preorder Tree: root - left - right
Inorder Tree: left - root - right
Postorder Tree: left - right - root
5.3:
Fast way to find k smallest element???
5.6: (Solved) 324 Wiggle Sort II(Medium)
5.8: 200 Number of Islands: in dfs function, what's the difference between if and while?
5.9: 279 how to do it in dp.
5.11: 560 Subarray Sum Equals K(Medium)
5.17:
399 Evaluate Division(Medium) (Solved)(Union find - Need more practice)
947 Most Stones Removed with Same Row or Column(Medium)
621 Task Scheduler(Medium)
(Solved) 54 Spiral Matrix(Medium)
393 UTF-8 Validation(Medium)
981 Time Based Key-Value Store(Medium)
974 Subarray Sums Divisible by K(Medium)
923 3Sum With Multiplicity(Medium)
5.19:
130???????
5.20:
when using backtracking, if there is something need to be substituted: 1.push 2:recursive 3:pop. if there is no, just recursive. Example: 17 vs 22.
Random number: a + rand() % n. a is the beginning, n is the range.
upper_bound, lower_bound: http://www.cnblogs.com/cobbliu/archive/2012/05/21/2512249.html
https://blog.csdn.net/qq_40160605/article/details/80150252
378??
5.21:
375. Guess Number Higher or Lower II
279: why it can save 2---ms because of self._dp?