Given preorder and inorder traversal of a tree, construct the binary tree.

Sort a linked list in O(n log n) time using constant space complexity.

分布式一致性协议通俗讲就是多机状态的一致性保障协议，即如果同一份状态存储在多台机器上，如何保证在多台机器上的状态彼此是一致的。一致是指读到的结果和写入的预期是一致的。

当然，如果度度熊使用第i个技能打在第j个怪兽上面的话，会使得第j个怪兽的生命值减少p[i]-b[j]，当然如果伤害小于防御，那么攻击就不会奏效。

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and put.

Write a function that takes an unsigned integer and returns the number of ’1' bits it has (also known as the Hamming weight).

近一年没有写代码了，手痒无聊，集中刷了近一百题，前一半按序全部做了，后面的挑了有意思的做，一眼看起来太简单的且重复类型的就忽略了。最近挑的题，发现没有了自测按钮，不太方便。暂时不再刷题了。开启新的一章，学习“机器学习”，从入门开始，看书、上公开课、做题练习、做笔记。

Given two words (beginWord and endWord), and a dictionary's word list, find the length of shortest transformation sequence from beginWord to endWord, such that:

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

Note:A solution using O(n) space is pretty straight forward. Could you devise a constant space solution?

Given two binary trees, write a function to check if they are equal or not.

Given a binary tree, determine if it is a valid binary search tree (BST).

Given s1, s2, s3, find whether s3 is formed by the interleaving of s1 and s2.

Given n, how many structurally unique BST's (binary search trees) that store values 1...n?

Given an integer n, generate all structurally unique BST's (binary search trees) that store values 1...n.

Given a binary tree, return the inorder traversal of its nodes' values.

Given a string containing only digits, restore it by returning all possible valid IP address combinations.

A message containing letters from A-Z is being encoded to numbers using the following mapping:

Given a collection of integers that might contain duplicates, nums, return all possible subsets.

Given two sorted integer arrays nums1 and nums2, merge nums2 into nums1 as one sorted array.

Given a string s1, we may represent it as a binary tree by partitioning it to two non-empty substrings recursively.

Given a 2D binary matrix filled with 0's and 1's, find the largest rectangle containing only 1's and return its area.

Given n non-negative integers representing the histogram's bar height where the width of each bar is 1, find the area of largest rectangle in the histogram.

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

Given a string S and a string T, find the minimum window in S which will contain all the characters in T in complexity O(n).

Given an array with n objects colored red, white or blue, sort them so that objects of the same color are adjacent, with the colors in the order red, white and blue.

Given a m x n matrix, if an element is 0, set its entire row and column to 0. Do it in place.

Given two words word1 and word2, find the minimum number of steps required to convert word1 to word2. (each operation is counted as 1 step.)