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14. Longest Common Prefix题目Write a function to find the longest common prefix string amongst an array of strings. If there is no common prefix, return an empty string "". Example 1: Input: strs = ["flower","flow","flight"] Output: "fl" Example 2: Input: strs = ["dog","racecar","car"] Output: "" Explanation: There is no common prefix among the input strings. Constraints: 1 <= strs.length...

10. Regular Expression MatchingGiven an input string s and a pattern p, implement regular expression matching with support for '.' and '*' where: '.' Matches any single character. '*' Matches zero or more of the preceding element. The matching should cover the entire input string (not partial). Example 1: 123Input: s = "aa", p = "a"Output: falseExplanation: "a" does not match the entire string "aa". Example...

9. Palindrome Number题目Determine whether an integer is a palindrome. An integer is a palindrome when it reads the same backward as forward. Example 1: 12Input: 121Output: true Example 2: 123Input: -121Output: falseExplanation: From left to right, it reads -121. From right to left, it becomes 121-. Therefore it is not a palindrome. Example 3: 123Input: 10Output: falseExplanation: Reads 01 from right to left. Therefore it is not a palindrome. Follow up: Coud you solve it without converting...

8. String to Integer (atoi)题目Implement the myAtoi(string s) function, which converts a string to a 32-bit signed integer (similar to C/C++'s atoi function). The algorithm for myAtoi(string s) is as follows: Read in and ignore any leading whitespace. Check if the next character (if not already at the end of the string) is '-' or '+'. Read this character in if it is either. This determines if the final result is negative or positive respectively. Assume the result is...

7. Reverse Integer题目Given a 32-bit signed integer, reverse digits of an integer. Example 1: Input: 123 Output: 321 Example 2: Input: -123 Output: -321 Example 3: Input: 120 Output: 21 **Note:**Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−2^31, 2^31 − 1]. For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows. 题目大意给出一个 32 位的有符号整数，需要将这个整数中每位上的数字进行反转。注意:假设我们的环境只能存储得下 32...

76. Minimum Window Substring题目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). Example: 12Input: S = "ADOBECODEBANC", T = "ABC"Output: "BANC" Note: If there is no such window in S that covers all characters in T, return the empty string "". If there is such window, you are guaranteed that there will always be only one unique minimum window in S. 题目大意给定一个源字符串 s，再给一个字符串...

904. Fruit Into Baskets题目In a row of trees, the i-th tree produces fruit with type tree[i]. You start at any tree of your choice, then repeatedly perform the following steps: Add one piece of fruit from this tree to your baskets. If you cannot, stop. Move to the next tree to the right of the current tree. If there is no tree to the right, stop. Note that you do not have any choice after the initial choice of starting tree: you must perform step 1, then step 2, then back to step 1, then...

12. Integer to Roman题目Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M. 12345678Symbol ValueI 1V 5X 10L 50C 100D 500M 1000 For example, 2 is written as II in Roman numeral, just two one's added together. 12 is written as XII, which is simply X + II. The number 27 is written as XXVII, which is XX + V + II. Roman numerals are usually written largest to smallest from...

105. Construct Binary Tree from Preorder and Inorder Traversal题目Given preorder and inorder traversal of a tree, construct the binary tree. **Note:**You may assume that duplicates do not exist in the tree. For example, given preorder = [3,9,20,15,7] inorder = [9,3,15,20,7] Return the following binary tree: 3 / \ 9 20 / \ 15 7 题目大意根据一棵树的前序遍历与中序遍历构造二叉树。 注意:你可以假设树中没有重复的元素。 解题思路 inorder_map：用于快速查找中序遍历中元素对应的索引，时间复杂度 O...

17. Letter Combinations of a Phone Number题目Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters. Example: 12Input: "23"Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"]. Note: Although...

73. Set Matrix Zeroes题目Given an *m* x *n* matrix. If an element is 0, set its entire row and column to 0. Do it in-place. Follow up: A straight forward solution using O(mn) space is probably a bad idea. A simple improvement uses O(m + n) space, but still not the best solution. Could you devise a constant space solution? Example 1: 12Input: matrix = [[1,1,1],[1,0,1],[1,1,1]]Output: [[1,0,1],[0,0,0],[1,0,1]] Example 2: 12Input: matrix = [[0,1,2,0],[3,4,5,2],[1,3,1,5]]Output:...

16. 3Sum Closest题目Given an array nums of n integers and an integer target, find three integers in nums such that the sum is closest to target. Return the sum of the three integers. You may assume that each input would have exactly one solution. Example: 123Given array nums = [-1, 2, 1, -4], and target = 1.The sum that is closest to the target is 2. (-1 + 2 + 1 = 2). 题目大意给定一个数组，要求在这个数组中找出 3 个数之和离 target 最近。 解题思路 初始值优化： 原代码中it初始化为...

11. Container With Most Water题目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 lines, which together with x-axis forms a container, such that the container contains the most water. Note: You may not slant the container and n is at least 2. The above vertical lines are represented by array [1,8,6,2,5,4,8,3,7]. In this case, the max area of...

1201. Ugly Number III题目Write a program to find the n-th ugly number. Ugly numbers are positive integers which are divisible by a or b or c. Example 1: Input: n = 3, a = 2, b = 3, c = 5 Output: 4 Explanation: The ugly numbers are 2, 3, 4, 5, 6, 8, 9, 10... The 3rd is 4. Example 2: Input: n = 4, a = 2, b = 3, c = 4 Output: 6 Explanation: The ugly numbers are 2, 3, 4, 6, 8, 9, 10, 12... The 4th is 6. Example 3: Input: n = 5, a = 2, b = 11, c = 13 Output: 10 Explanation: The ugly numbers are...

15. 3Sum题目Given an array nums of n integers, are there elements a, b, c in nums such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero. Note: The solution set must not contain duplicate triplets. Example: 1234567Given array nums = [-1, 0, 1, 2, -1, -4],A solution set is:[ [-1, 0, 1], [-1, -1, 2]] 题目大意给定一个数组，要求在这个数组中找出 3 个数之和为 0 的所有组合。 解题思路用 map 提前计算好任意 2 个数字之和，保存起来，可以将时间复杂度降到...

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)....

977. Squares of a Sorted ArrayGiven an array of integers A sorted in non-decreasing order, return an array of the squares of each number, also in sorted non-decreasing order. Example 1: 12Input: [-4,-1,0,3,10]Output: [0,1,9,16,100] Example 2: 12Input: [-7,-3,2,3,11]Output: [4,9,9,49,121] Note: 1 <= A.length <= 10000 -10000 <= A[i] <= 10000 A is sorted in non-decreasing...

844. Backspace String Compare题目Given two strings S and T, return if they are equal when both are typed into empty text editors. # means a backspace character. Example 1: 123Input: S = "ab#c", T = "ad#c"Output: trueExplanation: Both S and T become "ac". Example 2: 123Input: S = "ab##", T = "c#d#"Output: trueExplanation: Both S and T become "". Example 3: 123Input: S = "a##c", T = "#a#c"Output: trueExplanation:...

922. Sort Array By Parity IIGiven an array of integers nums, half of the integers in nums are odd, and the other half are even. Sort the array so that whenever nums[i] is odd, i is odd, and whenever nums[i] is even, i is even. Return any answer array that satisfies this condition. Example 1: 123Input: nums = [4,2,5,7]Output: [4,5,2,7]Explanation: [4,7,2,5], [2,5,4,7], [2,7,4,5] would also have been accepted. Example 2: 12Input: nums = [2,3]Output: [2,3] 双指针解法： 初始化： 偶数索引指针 i 从 0 开始，每次移动 2...

905. Sort Array By ParityGiven an integer array nums, move all the even integers at the beginning of the array followed by all the odd integers. Return any array that satisfies this condition. Example 1: 123Input: nums = [3,1,2,4]Output: [2,4,3,1]Explanation: The outputs [4,2,3,1], [2,4,1,3], and [4,2,1,3] would also be accepted. Example 2: 12Input: nums = [0]Output: [0] 解法1：双指针12345678910111213141516class Solution{public: vector<int> sortArrayByParity(vector<int>...

1089. Duplicate Zeros题目Given a fixed length array arr of integers, duplicate each occurrence of zero, shifting the remaining elements to the right. Note that elements beyond the length of the original array are not written. Do the above modifications to the input array in place, do not return anything from your function. Example 1: 123Input: [1,0,2,3,0,4,5,0]Output: nullExplanation: After calling your function, the input array is modified to: [1,0,0,2,3,0,0,4] Example 2: 123Input:...

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; //...

26. Remove Duplicates from Sorted Array给你一个 非严格递增排列 的数组 nums ，请你** 原地** 删除重复出现的元素，使每个元素 只出现一次 ，返回删除后数组的新长度。元素的 相对顺序 应该保持 一致 。然后返回 nums 中唯一元素的个数。 考虑 nums 的唯一元素的数量为 k ，你需要做以下事情确保你的题解可以被通过： 更改数组 nums ，使 nums 的前 k 个元素包含唯一元素，并按照它们最初在 nums 中出现的顺序排列。nums 的其余元素与 nums 的大小不重要。 返回 k 。 判题标准: 系统会用下面的代码来测试你的题解: 123456789int[] nums = [...]; // 输入数组int[] expectedNums = [...]; // 长度正确的期望答案int k = removeDuplicates(nums); // 调用assert k == expectedNums.length;for (int i = 0; i < k; i++) { assert...

5. Longest Palindromic Substring题目Given a string s, return the longest palindromic substring in s. Example 1: 1234Input: s = "babad"Output: "bab"Note: "aba" is also a valid answer. Example 2: 123Input: s = "cbbd"Output: "bb" Example 3: 123Input: s = "a"Output: "a" Example 4: 123Input: s = "ac"Output: "a" Constraints: 1 <= s.length <= 1000 s consist of only digits and English letters...

704. Binary Search题目Given a sorted (in ascending order) integer array nums of n elements and a target value, write a function to search target in nums. If target exists, then return its index, otherwise return -1. Example 1: Input: nums = [-1,0,3,5,9,12], target = 9 Output: 4 Explanation: 9 exists in nums and its index is 4 Example 2: Input: nums = [-1,0,3,5,9,12], target = 2 Output: -1 Explanation: 2 does not exist in nums so return -1 Note: You may assume that all elements in nums are...

509. Fibonacci Number题目The Fibonacci numbers, commonly denoted F(n) form a sequence, called the Fibonacci sequence, such that each number is the sum of the two preceding ones, starting from 0 and 1. That is, F(0) = 0,   F(1) = 1 F(N) = F(N - 1) + F(N - 2), for N > 1. Given N, calculate F(N). Example 1: Input: 2 Output: 1 Explanation: F(2) = F(1) + F(0) = 1 + 0 = 1. Example 2: Input: 3 Output: 2 Explanation: F(3) = F(2) + F(1) = 1 + 1 = 2. Example 3: Input: 4 Output: 3 Explanation: F(4)...

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...

162. Find Peak Element题目A peak element is an element that is greater than its neighbors. Given an input array nums, where nums[i] ≠ nums[i+1], find a peak element and return its index. The array may contain multiple peaks, in that case return the index to any one of the peaks is fine. You may imagine that nums[-1] = nums[n] = -∞. Example 1: Input: nums = [1,2,3,1] Output: 2 Explanation: 3 is a peak element and your function should return the index number 2. Example 2: Input: nums =...

72. Edit DistanceGiven two strings word1 and word2, return the minimum number of operations required to convert word1 to word2. You have the following three operations permitted on a word: Insert a character Delete a character Replace a character Example 1: 123456Input: word1 = "horse", word2 = "ros"Output: 3Explanation: horse -> rorse (replace 'h' with 'r')rorse -> rose (remove 'r')rose -> ros (remove 'e') Example...

70. Climbing Stairs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? Note: Given n will be a positive integer. Example 1: Input: 2 Output: 2 Explanation: There are two ways to climb to the top. 1. 1 step + 1 step 2. 2 steps Example 2: Input: 3 Output: 3 Explanation: There are three ways to climb to the top. 1. 1 step + 1 step + 1 step 2. 1 step + 2 steps 3. 2 steps + 1...

69. Sqrt(x)Given a non-negative integer x, return the square root of x rounded down to the nearest integer. The returned integer should be non-negative as well. You must not use any built-in exponent function or operator. For example, do not use pow(x, 0.5) in c++ or x ** 0.5 in python. Example 1: 123Input: x = 4Output: 2Explanation: The square root of 4 is 2, so we return 2. Example 2: 123Input: x = 8Output: 2Explanation: The square root of 8 is 2.82842..., and since we round it down to...

35. Search Insert Position题目Given a sorted array and a target value, return the index if the target is found. If not, return the index where it would be if it were inserted in order. You may assume no duplicates in the array. Example 1: 12Input: [1,3,5,6], 5Output: 2 Example 2: 12Input: [1,3,5,6], 2Output: 1 Example 3: 12Input: [1,3,5,6], 7Output: 4 Example 4: 12Input: [1,3,5,6], 0Output:...

34. Find First and Last Position of Element in Sorted Array题目Given an array of integers nums sorted in ascending order, find the starting and ending position of a given target value. Your algorithm's runtime complexity must be in the order of O(log n). If the target is not found in the array, return [-1, -1]. Example 1: Input: nums = [5,7,7,8,8,10], target = 8 Output: [3,4] Example 2: Input: nums = [5,7,7,8,8,10], target = 6 Output: [-1,-1] 查找第一个出现的位置： 当找到目标值时，不立即返回，而是继续向左查找（right...

33. Search in Rotated Sorted Array题目Suppose an array sorted in ascending order is rotated at some pivot unknown to you beforehand. (i.e., [0,1,2,4,5,6,7] might become [4,5,6,7,0,1,2]). You are given a target value to search. If found in the array return its index, otherwise return -1. You may assume no duplicate exists in the array. Your algorithm's runtime complexity must be in the order of O(log n). Example 1: Input: nums = [4,5,6,7,0,1,2], target = 0 Output: 4 Example 2: Input: nums...

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...

27. Remove Element给你一个数组 nums 和一个值 val，你需要 原地 移除所有数值等于 val 的元素。元素的顺序可能发生改变。然后返回 nums 中与 val 不同的元素的数量。 假设 nums 中不等于 val 的元素数量为 k，要通过此题，您需要执行以下操作： 更改 nums 数组，使 nums 的前 k 个元素包含不等于 val 的元素。nums 的其余元素和 nums 的大小并不重要。 返回 k。 用户评测： 评测机将使用以下代码测试您的解决方案： 123456789101112int[] nums = [...]; // 输入数组int val = ...; // 要移除的值int[] expectedNums = [...]; // 长度正确的预期答案。 // 它以不等于 val 的值排序。int k = removeElement(nums, val); // 调用你的实现assert k == expectedNums.length;sort(nums, 0, k); // 排序...

4. Median of Two Sorted Arrays题目There are two sorted arrays nums1 and nums2 of size m and n respectively. Find the median of the two sorted arrays. The overall run time complexity should be O(log (m+n)). You may assume nums1 and nums2 cannot be both empty. Example 1: nums1 = [1, 3] nums2 = [2] The median is 2.0 Example 2: nums1 = [1, 2] nums2 = [3, 4] The median is (2 + 3)/2 = 2.5 题目大意给定两个大小为 m 和 n 的有序数组 nums1 和 nums2。 请你找出这两个有序数组的中位数，并且要求算法的时间复杂度为 O(log(m +...

3. Longest Substring Without Repeating Characters题目Given a string, find the length of the longest substring without repeating characters. Example 1: 123Input: "abcabcbb"Output: 3 Explanation: The answer is "abc", with the length of 3. Example 2: 123Input: "bbbbb"Output: 1Explanation: The answer is "b", with the length of 1. Example 3: 1234Input: "pwwkew"Output: 3Explanation: The answer is "wke", with the length of 3. ...

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 zeros, 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...

1. Two Sum题目Given an array of integers, return indices of the two numbers such that they add up to a specific target. You may assume that each input would have exactly one solution, and you may not use the same element twice. Example: 1234Given nums = [2, 7, 11, 15], target = 9,Because nums[0] + nums[1] = 2 + 7 = 9,return [0, 1]. 题目大意在数组中找到两个数，它们的和等于给定的目标值 target，并返回这两个数在数组中的下标。要求每个元素只能使用一次，且保证有且仅有一个解。 解题思路方法一：双层循环（暴力枚举）思路 通过双重循环遍历数组中的每一对元素，检查它们的和是否等于目标值 target。外层循环控制第一个元素的索引...