Leetcode

461. 汉明距离两个整数之间的 汉明距离 指的是这两个数字对应二进制位不同的位置的数目。 给你两个整数 x 和 y，计算并返回它们之间的汉明距离。 示例 1： 1234567输入：x = 1, y = 4输出：2解释：1 (0 0 0 1)4 (0 1 0 0) ↑ ↑上面的箭头指出了对应二进制位不同的位置。 示例 2： 12输入：x = 3, y = 1输出：1 题目大意 汉明距离是指两个整数的二进制表示中，对应位置二进制位不同的数目。给定两个整数 x 和 y，计算并返回它们之间的汉明距离。 例如： 输入 x=1, y=4，二进制分别为 0001 和 0100，对应位不同的位置有 2 处，输出 2； 输入 x=3, y=1，二进制分别为 0011 和 0001，对应位不同的位置有 1 处，输出 1。 解题思路核心思路是利用异或运算 + 统计 1 的个数，步骤如下： 异或运算（XOR）：异或的特性是 “相同为 0，不同为 1”。对 x 和 y 做异或操作，得到的结果 xor_result 中，每一位的 1 都对应 x 和 y 二进制位不同的位...

2. Add Two Numbers题目You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list. You may assume the two numbers do not contain any leading zero, except the number 0 itself. Example 1: 123Input: l1 = [2,4,3], l2 = [5,6,4]Output: [7,0,8]Explanation: 342 + 465 = 807. Example 2: 12Input: l1 = [0], l2 = [0]Output: [0] Example 3: 12I...

455. Assign CookiesAssume you are an awesome parent and want to give your children some cookies. But, you should give each child at most one cookie. Each child i has a greed factor g[i], which is the minimum size of a cookie that the child will be content with; and each cookie j has a size s[j]. If s[j] >= g[i], we can assign the cookie j to the child i, and the child i will be content. Your goal is to maximize the number of your content children and output the maximum number. Example 1: 1...

1. Two Sum题目Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target. You may assume that each input would have exactly one solution, and you may not use the same element twice. You can return the answer in any order. Example 1: 123Input: nums = [2,7,11,15], target = 9Output: [0,1]Explanation: Because nums[0] + nums[1] == 9, we return [0, 1]. Example 2: 12Input: nums = [3,2,4], target = 6Output: [1,2] Example 3: 12Input: nums =...

454. 4Sum IIGiven four integer arrays nums1, nums2, nums3, and nums4 all of length n, return the number of tuples (i, j, k, l) such that: 0 <= i, j, k, l < n nums1[i] + nums2[j] + nums3[k] + nums4[l] == 0 Example 1: 123456Input: nums1 = [1,2], nums2 = [-2,-1], nums3 = [-1,2], nums4 = [0,2]Output: 2Explanation:The two tuples are:1. (0, 0, 0, 1) -> nums1[0] + nums2[0] + nums3[0] + nums4[1] = 1 + (-2) + (-1) + 2 = 02. (1, 1, 0, 0) -> nums1[1] + nums2[1] + nums3[0] + nums4[0] = 2 +...

445. Add Two Numbers IIYou are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list. You may assume the two numbers do not contain any leading zero, except the number 0 itself. 1234567891011121314151617181920212223242526272829303132333435363738394041424344/** * Definition for singly-linked list. * struct ListNode { * int v...

429. N-ary Tree Level Order TraversalGiven an n-ary tree, return the level order traversal of its nodes' values. Nary-Tree input serialization is represented in their level order traversal, each group of children is separated by the null value (See examples). Example 1: 12Input: root = [1,null,3,2,4,null,5,6]Output: [[1],[3,2,4],[5,6]] Example 2: 12Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]Output: [[1],[2,3,4,5],[6,7,8,9,10]...

409. 最长回文串给定一个包含大写字母和小写字母的字符串 s ，返回 通过这些字母构造成的 最长的 回文串 的长度。 在构造过程中，请注意 区分大小写 。比如 "Aa" 不能当做一个回文字符串。 示例 1: 1234输入:s = "abccccdd"输出:7解释:我们可以构造的最长的回文串是"dccaccd", 它的长度是 7。 示例 2: 123输入:s = "a"输出:1解释：可以构造的最长回文串是"a"，它的长度是 1。 解题思路核心思路是统计字符频率，利用回文串的特性计算最大长度： 回文串特性： 回文串对称位置的字符相同； 偶数长度的回文串：所有字符出现次数均为偶数； 奇数长度的回文串：仅有一个字符出现奇数次（位于中心），其余均为偶数次。 频率统计： 统计字符串中每个字符的出现次数； 累计所有字符的最大偶数次（例如，出现 5 次的字符可贡献 4 次）； 若存在出现奇数次的字符，可在结果中加 1（作为中心字符）。 计算逻辑： 初始化结果 max_len...

404. Sum of Left LeavesGiven the root of a binary tree, return the sum of all left leaves. A leaf is a node with no children. A left leaf is a leaf that is the left child of another node. Example 1: 123Input: root = [3,9,20,null,null,15,7]Output: 24Explanation: There are two left leaves in the binary tree, with values 9 and 15 respectively. Example 2: 12Input: root = [1]Output: 0 题目大意给定一棵二叉树的根节点 root，返回所有左叶子的节点值之和。左叶子的定义是：既是叶子节点（没有子节点），又是其父节点的左子节点。 例如： 输入二叉树 [3,9,20,null,null,15,7]，左叶子为 ...

402. 移掉 K 位数字给你一个以字符串表示的非负整数 num 和一个整数 k ，移除这个数中的 k 位数字，使得剩下的数字最小。请你以字符串形式返回这个最小的数字。 示例 1 ： 123输入：num = "1432219", k = 3输出："1219"解释：移除掉三个数字 4, 3, 和 2 形成一个新的最小的数字 1219 。 示例 2 ： 123输入：num = "10200", k = 1输出："200"解释：移掉首位的 1 剩下的数字为 200. 注意输出不能有任何前导零。 示例 3 ： 123输入：num = "10", k = 2输出："0"解释：从原数字移除所有的数字，剩余为空就是 0 。 解题思路核心思路是单调栈 + 贪心算法，通过维护一个递增的栈来确保剩余数字最小，同时控制移除的数字数量不超过 k： 单调栈逻辑： 遍历字符串中的每个数字，对于当前数字c： 若栈不为空，且栈顶数字大于 c，且仍有可移除的次数（k > 0），则弹...

367. Valid Perfect Square题目Given a positive integer num, write a function which returns True if num is a perfect square else False. Note: Do not use any built-in library function such as sqrt. Example 1: Input: 16 Output: true Example 2: Input: 14 Output: false 完全平方数可以表示为连续奇数的和。 算法原理该算法利用了以下数学特性： 1 = 1（1²） 1 + 3 = 4（2²） 1 + 3 + 5 = 9（3²） 1 + 3 + 5 + 7 = 16（4²） 以此类推，n² = 1 + 3 + 5 + ... + (2n-1) 算法通过不断从目标数中减去连续的奇数（1, 3, 5, 7...），如果最终结果恰好为 0，则说明该数是完全平方数。 123456789...

349. Intersection of Two ArraysGiven two integer arrays nums1 and nums2, return an array of their intersection. Each element in the result must be unique and you may return the result in any order. Example 1: 12Input: nums1 = [1,2,2,1], nums2 = [2,2]Output: [2] Example 2: 123Input: nums1 = [4,9,5], nums2 = [9,4,9,8,4]Output: [9,4]Explanation: [4,9] is also accepted. 题目大意给定两个整数数组 nums1 和 nums2，返回它们的交集。结果中的每个元素必须是唯一的，并且可以按任意顺序返回。 解题思路可以使用哈希集合来高效求解： 先将第一个数组中的元素存入一个哈希集合，自动去除重复元素 遍历第二个数组，检查每个元...

316. 去除重复字母给你一个字符串 s ，请你去除字符串中重复的字母，使得每个字母只出现一次。需保证 返回结果的字典序最小（要求不能打乱其他字符的相对位置）。 示例 1： 12输入：s = "bcabc"输出："abc" 示例 2： 12输入：s = "cbacdcbc"输出："acdb" 核心思路是单调栈 + 贪心算法，通过维护一个单调递增的栈来确保字典序最小，同时利用计数数组判断字符是否还有剩余，确保每个字符只保留一次： 预处理： 统计字符串中每个字符的出现次数（count 数组）； 记录字符是否已在栈中（in_stack 数组），避免重复添加。 单调栈逻辑： 遍历字符串中的每个字符C： 减少 count[c]（当前字符已被考虑）； 若 c 已在栈中，直接跳过； 若 c 不在栈中，且栈不为空，且栈顶字符大于 c，且栈顶字符在后续还有出现（count[栈顶字符] > 0），则弹出栈顶字符（确保字典序更小）； 将 c 压入栈，并标记为已在栈中。 结果构建： 栈中字符即为去重...

287. Find the Duplicate NumberGiven an array of integers nums containing n + 1 integers where each integer is in the range [1, n] inclusive. There is only one repeated number in nums, return this repeated number. You must solve the problem without modifying the array nums and using only constant extra space. Example 1: 12Input: nums = [1,3,4,2,2]Output: 2 Example 2: 12Input: nums = [3,1,3,4,2]Output: 3 Example 3: 12Input: nums = [3,3,3,3,3]Output: 3 题目大意给定一个包含 n + 1 个整数的数组 nums，其中每个整数都在 [1...

283. Move Zeroes题目Given an array nums, write a function to move all 0's to the end of it while maintaining the relative order of the non-zero elements. Example 1: 12Input: [0,1,0,3,12]Output: [1,3,12,0,0] Note: You must do this in-place without making a copy of the array. Minimize the total number of operations. 解法1：双指针1234567891011121314151617181920class Solution {public: void moveZeroes(vector<int>& nums) { int n = nums.size(); int i = 0; // 慢指针，记录下...

257. Binary Tree PathsGiven the root of a binary tree, return all root-to-leaf paths in any order. A leaf is a node with no children. Example 1: 12Input: root = [1,2,3,null,5]Output: ["1->2->5","1->3"] Example 2: 12Input: root = [1]Output: ["1"] 题目大意给定一棵二叉树的根节点 root，返回所有从根节点到叶子节点的路径。叶子节点是指没有子节点的节点，路径以字符串形式表示，节点值之间用 "->" 连接。 例如： 输入二叉树 [1,2,3,null,5]，根到叶子的路径为 1->2->5 和 1->3，返回 ["1->2->5","1->3"]。 解题思...

239. Sliding Window MaximumYou are given an array of integers nums, there is a sliding window of size k which is moving from the very left of the array to the very right. You can only see the k numbers in the window. Each time the sliding window moves right by one position. Return the max sliding window. Example 1: 1234567891011Input: nums = [1,3,-1,-3,5,3,6,7], k = 3Output: [3,3,5,5,6,7]Explanation: Window position Max--------------- -----[1 3 -1] -3 5 3 6 ...

232. Implement Queue using StacksImplement a first in first out (FIFO) queue using only two Stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty). Implement the MyQueue class: void push(int x) Pushes element x to the back of the queue. int pop() Removes the element from the front of the queue and returns it. int peek() Returns the element at the front of the queue. boolean empty() Returns true if the queue is empty, false otherwise. No...

227. Basic Calculator IIGiven a string s which represents an expression, evaluate this expression and return its value. The integer division should truncate toward zero. You may assume that the given expression is always valid. All intermediate results will be in the range of [-231, 231 - 1]. Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval(). 题目大意给定一个字符串 s 表示一个算术表达式，要求计算该表达式的值并返回。表达式仅包含非负整数、+、-、*、/ 四种运算符，以及可能的空格。整数除法需要向...

226. Invert Binary TreeGiven the root of a binary tree, invert the tree, and return its root. Example 1: 12Input: root = [4,2,7,1,3,6,9]Output: [4,7,2,9,6,3,1] Example 2: 12Input: root = [2,1,3]Output: [2,3,1] Example 3: 12Input: root = []Output: [] 题目大意给定一棵二叉树的根节点 root，翻转这棵二叉树（即交换每个节点的左子树和右子树），并返回翻转后的根节点。 例如： 输入二叉树 [4,2,7,1,3,6,9]，翻转后每个节点的左右子树互换，输出为 [4,7,2,9,6,3,1]。 解题思路翻转二叉树的核心是交换每个节点的左子节点和右子节点，可以通过递归或迭代两种方式实现： 1. 递归法利用二叉树的递归性质： 若当前节点为空，直接返回； 否则，先交换当前节点的左、右子节点； 递归翻转当前节点的左子树和右子树。 2...

225. Implement Stack using QueuesImplement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty). Implement the MyStack class: void push(int x): Pushes element x to the top of the stack. int pop(): Removes the element on the top of the stack and returns it. int top(): Returns the element on the top of the stack. boolean empty(): Returns true if the stack is empty, false otherwise. Notes: ...

222. Count Complete Tree NodesGiven the root of a complete binary tree, return the number of the nodes in the tree. According to Wikipedia, every level, except possibly the last, is completely filled in a complete binary tree, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes inclusive at the last level h. Design an algorithm that runs in less than O(n) time complexity. Example 1: 12Input: root = [1,2,3,4,5,6]Output: 6 Example 2: 12Input: root ...

216. Combination Sum IIIFind all valid combinations of k numbers that sum up to n such that the following conditions are true: Only numbers 1 through 9 are used. Each number is used at most once. Return a list of all possible valid combinations. The list must not contain the same combination twice, and the combinations may be returned in any order. Example 1: 12345Input: k = 3, n = 7Output: [[1,2,4]]Explanation:1 + 2 + 4 = 7There are no other valid combinations. Example 2: 1234567Input: k ...

209. Minimum Size Subarray Sum题目Given an array of n positive integers and a positive integer s, find the minimal length of a contiguous subarray of which the sum ≥ s. If there isn't one, return 0 instead. Example 1: 123Input: s = 7, nums = [2,3,1,2,4,3]Output: 2Explanation: the subarray [4,3] has the minimal length under the problem constraint. Follow up: If you have figured out the O(n) solution, try coding another solution of which the time complexity is O(n log n). 解题思路这一题的解题思...

206. Reverse Linked ListGiven the head of a singly linked list, reverse the list, and return the reversed list. Example 1: 12Input: head = [1,2,3,4,5]Output: [5,4,3,2,1] Example 2: 12Input: head = [1,2]Output: [2,1] Example 3: 12Input: head = []Output: [] Constraints: The number of nodes in the list is the range [0, 5000]. -5000 <= Node.val <= 5000 解法 1：迭代解法123456789101112131415161718192021222324252627/** * Definition for singly-linked list. * struct ListNode { * int v...