練手題：
254. Factor Combinations *
261. Graph Valid Tree
264. Ugly Number II
277. Find the Celebrity
267. Palindrome Permutation II
294. Flip Game II
306. Additive Number
310. Minimum Height Trees
314. Binary Tree Vertical Order Traversal
311. Sparse Matrix Multiplication
318. Maximum Product of Word Lengths
319. Bulb Switcher
不會題：
255. Verify Preorder Sequence in Binary Search Tree: 這題就是要把preorder變成inorder，然后還有一個follow up，感覺更難了 *
271. Encode and Decode Strings: 找找更高效率的答案畔乙，看來看去我覺得我的解法挺好弹囚，pass這題
284. Peeking Iterator: 好好的去理解iterator的含義 *
300. Longest Increasing Subsequence: 仔細得看一下follow up的問題 *
309. Best Time to Buy and Sell Stock with Cooldown: 正確的構造出來state的transfer function是關鍵 *
320. Generalized Abbreviation: 感覺是一道萬年不會的題 *
