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Reverse digits of an integer. Example1: x = 123, return 321 Example2: x = -123, return -321Have you thought about this? Here are some good questions to ask before coding. Bonus points for you if

78. SubsetsGiven a set of distinct integers, nums, return all possible subsets. 给定一组非重复数字，求出所有可能的子集解析：例如 [1,2,3]，解法： 首先放[]，然后往已有的[]中放1 1. 首先放1 此时已有[ [], 1 ] 2. 然后对[ [], 1 ] 放2 于是此时有 [ [], [1], [2

1. Leet Code OJ 2. Add Two NumbersYou are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit.

树问题规律总结出现树问题，多数情况都是需要递归遍历（dfs和bfs），并且要根据题目要求，来思考相关的树的性质： 二叉树的一些性质问题汇总1. Leet Code OJ 101. Symmetric Tree程序：/** * Definition for a binary tree node. * public class TreeNode { * int val; * T

问题：给出一个网段，该网段的地址都属于黑名单，验证其他ip地址是否属于黑名单要想到通过二进制的位运算来实现：ip & 子网掩码 = 网段对于一个CIDR的ip地址，怎么得到子网掩码？ 可以得到CIDR中的网络号位数netCount，然后：int mask = 0xFFFFFFFF << (32 - netCount);这样就能得到子网掩码public class IPFilter { /**

1. 微信红包（2016腾讯）给定一个红包的金额数组gifts及它的大小n，请返回所求红包的金额。 若没有金额超过总数的一半，返回0输入： [1,2,3,2,2],5输出： 2解答：可以先把数组排序，如果一个数的出现次数超过一半，排序后必然出现在中间2. 删除重复字符（2016华为）每组数据一行，按字符串原有的字符顺序，输出字符集合，即重复出现并靠后的字母不输出。输入： abcqweracb

题目You are climbing a stair case. It takes n steps to reach to the top.Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?这是一个爬楼梯问题，给定楼梯层数n，每一步只能走1层或者2层，问上到楼

总结栈问题通常和对称联系起来，一个出现，必须和另一个出现才合法，例如，出现“（”，就要出现“）”才合法，当遇到对称的问题时，可以考虑是不是栈问题1. Leet Code Oj 20. Valid ParenthesesGiven a string containing just the characters ‘(‘, ‘)’, ‘{‘, ‘}’, ‘[’ and ‘]’, determine if

Write a function to find the longest common prefix string amongst an array of strings.编写一个函数来查找字符串数组中最长的公共前缀字符串。代码public String longestCommonPrefix(String[] strs) { if(strs == null || strs.length =

Given a digit string, return all possible letter combinations that the number could represent.A mapping of digit to letters (just like on the telephone buttons) is given below.Input:Digit string “23”

问题：Given n non-negative integers a1, a2, …, an, where each represents a point at coordinate (i, ai). n vertical lines are drawn such that the two endpoints of line i is at (i, ai) and (i, 0). Find two

344. Reverse StringWrite a function that takes a string as input and returns the string reversed.Example: Given s = “hello”, return “olleh”.最简单的做法new StringBuilder(s).reverse().toString();更高效的做法public