练手题：
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
不会题：
255. Verify Preorder Sequence in Binary Search Tree: 这题就是要把preorder变成inorder，然后还有一个follow up，感觉更难了 *
271. Encode and Decode Strings: 找找更高效率的答案，看来看去我觉得我的解法挺好，pass这题
284. Peeking Iterator: 好好的去理解iterator的含义 *
300. Longest Increasing Subsequence: 仔细得看一下follow up的问题 *
309. Best Time to Buy and Sell Stock with Cooldown: 正确的构造出来state的transfer function是关键 *
320. Generalized Abbreviation: 感觉是一道万年不会的题 *
