痛定思痛，不忘初心

day 8: binary search
k closest in sorted array: find closest + sliding window
search in unknown sized sorted array
total occurrence: combine first occur and find closest
search in sorted matrix: 如果用left=mid/right=mid就用while left<right-1，防止死循环

day 9:
sorting: bubble/select/insert/merge/quick/rainbow(竟然是我fb题)
reverse linked list: recursive/iterative

day 10:
merge two sorted linked lists
linked list: selection/merge/quick sort：quicksort花了几小时，partition里面要分别处理大于和小于两种情况，应该返回head, head_prev, head_nxt, pivot, pivot_prev, pivot_nxt

day 11:
coding 1h
tree: iterative inorder/preorder/postorder（inorder和postorder做法类似，stack元素是tuple(node, count)）
stack: valid parenthese, evaluate reverse polish notation（import operator简化美观很多）

Data science 3h
AB testing
hypothese testing

day 12:
balanced binary tree, symmetric binary tree, BST: 都用helper function实现divide & conquer （left/right subtree）
fibonacci number
reverse linked list, in pairs: recursive/iterative
n queens: dfs recursive，重点在判断左右对角线上的点，path length-》row, index->column
lowest common ancestor (exist)：recursive dfs先遇到的即lca
lowest common ancestor (may not exist, w/ parent node)：用parent node dfs把所有parent存下来，再dfs另一个node时返回在存下来的祖先集合里的节点；优化space：两个节点拉到相同高度同时dfs back找共同parent

day 13:
merge k sorted arrays by heap: space O(k)
smallest range: heap space O(k)
perfect squares: BFS, 检查所有小于本身的平方数，递减下去，返回最快减到0的层数
daily temperatures: 用stack，存入递减的index，遇到递增pop所有小于当前数的index，time: O(n)
summary ranges: 处理各种corner case

day 14:
al anagrams: hashmap and match counter
upside down tree: similar to reverse linked list
max difference of the tree: divide and conquer
k smallest: min heap or max heap
reverse w/o vowel: two pointer
top k frequent: hashmap

day 15: gg的题都是各种拐弯抹角
226 invert binary tree: iterative, bottom up/top down recursive
399 evaluate division: graph + bfs
684 redundant connection: union find
777 swap adjacent in LR string: two pointer
844 backspace string compare: skip and compare

楼主加微信？ abcde123abcde45

补打day 16
practice union find: find(x) is to find x's root ancestor; union(x,y) is to add y as x's ancestor
number of islands
friend circles
redundant connection
accounts merge
most stones removed with same row or column
regions cut by slashes
insert delete getRandom

少刷一天：（

补打day 17
周六contest 1051-1054 3mdium 1 easy 不难，题目越来越不直接，还挺巧妙的
tree min depth