[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_3 (8/15)

(1) Find All Anagrams in a String
用 int[char] 来记录 sliding window 里每个char 的个数， 用 absSum 来判断是不是参照 word 的 anagram

(2) Word Abbreviation
记录第几轮的 abbreviation. 用 两个 hashmap 循环去保证每个word 有 unique abbreviation

#Day_15

Finished * 2
250. Count Univalue Subtrees
415. Add Strings

Day 10
127, word ladder

Day 11 only read through some notes, but did not code at all...

Day 12
1. 1086, maxheap
2. 310, 490, 505, 675, 909, bfs
3. 973, sort or heap

day 13
Validate Binary Search Tree
Binary Tree Vertical Order Traversal

Day_3 (8/16)
Convert BST to Greater Tree
Find leaves of Binary Tree

#Day_16

Finished * 2
528. Random Pick with Weight，题本身简单，题目看了半天
340. Longest Substring with At Most K Distinct Characters 题本身很简单，debug了半天

#Day_5
1160. Find Words That Can Be Formed by Characters
1161. Maximum Level Sum of a Binary Tree

Day 13
1. 419 battleship, pay attention to the condition to simplify the solution even more
2. 78 subsets, dfs & bfs
3. 785, is graph bipartite, dfs & bfs => use states[] or colors[] to record the group the node belongs to
4. 721, accounts merge, union find, and such a tedious code
5. 36, valid sudoku, smart way to record your traversal
6. 37, sudoku solver, dfs + backtracking (do not try to be smart when solving the interview problems...)

刷题day k, 打卡day1:

157. Read N Characters Given Read4 - Easy question but confusing description
161. One Edit Distance - Simple string comparison
199. Binary Tree Right Side View - Same as BST level order traversal (DFS or BFS)
208. Implement Trie (Prefix Tree) - Important data structure. Easy to implement in Python using nested dictionaries
211. Add and Search Word - Data structure design - Using trie. Note that need to do a DFS to exhaust all possibilities for `.`
39. Combination Sum - classic backtracking
40. Combination Sum II - classic backtracking
216. Combination Sum III - classic backtracking
377. Combination Sum IV - bottom-up DP. For the follow up, might need to restrain length of combination. E.g. nums = [-1, 1], target = 0. Then there are infinite combinations.

day 14
复习一题post-order，然后做了5个BST相关
postorder traversal 用双stack
Largest BST Subtree
Serialize and Deserialize BST
426. Convert Binary Search Tree to Sorted Doubly Linked List
Convert Sorted Array to Binary Search Tree
Convert Sorted List to Binary Search Tree