[第二期活动] 在职刷题打卡，目标工作日一天2题，周末一天5题

#Day_20

Finished * 2
Count Complete Tree Nodes
Simplify Path

补充内容 (2019-8-22 10:21):
多做一道
Degree of an Array

补充内容 (2019-8-22 13:20):
看TI顺便刷了点
Maximum Distance in Arrays
Reviewed
Best Time to Buy and Sell Stock
Best Time to Buy and Sell Stock II
Best Time to Buy and Sell Stock III

Day_9 (8/21)
Search a 2D Matrix II // exclude from left-bottom to right-top
First bad version

day 18
Count Univalue Subtrees 用了一把刚学的骚操作
Next Greater Element I

Day 16 Continue on tree
114 flatten binary tree to a linked list, dfs, remember to set the left to nil or Moris? traversal (iteratively, still need some time to consume the ideas)
124 binary tree maximum path sum, dfs, return single path sum and max, the path sum will either be a single path, or a comb of the current node and left and right child path sum
238 product of array except self, two pass, T: O(n), S: O(1)
261 graph valid tree, construct graph, either use bfs/dfs to check if every node is visited or use dfs to detect if there is a cycle
  - according to graph theory, in order to be a valid tree, three conditions are to be met: 1. n nodes w/ n - 1 edges 2. every node is connected 3. no cycle; If two out of three are met, the third one will be met as well.
938 range sum of bst, dfs

#Day_7
347. Top-K-Frequent Element
78. Subsets

#Day_21

Finished * 2
LFU Cache
Cherry Pickup, 3D DP

Day_10 (8/22)

Knight shortest path
Pacific Atlantic Water Flow

day 19
新做两题：
Count Complete Tree Nodes
Minimum Distance Between BST Nodes
这两个换了方法又写了一遍：
Serialize and Deserialize BST
Next Greater Element I

Day 1 刷educative.io coderust
1. Find the Smallest Common Number from 3 arrays with ascending orders
2. rotate array
3. find the low/high index given a sorted array
4. move zeros to left given an unsorted array
5. find maximum single sell profit from an array of stock cost

#Day_8

34.find-first-and-last-position-of-element-in-sorted-array
14.longest-common-prefix