Leetcode 0045. 跳跃游戏 II
45. 跳跃游戏 II给定一个长度为 n 的 0 索引整数数组 nums。初始位置为 nums[0]。 每个元素 nums[i] 表示从索引 i 向后跳转的最大长度。换句话说，如果你在索引 i 处，你可以跳转到任意 (i + j) 处： 0 <= j <= nums[i] 且 i + j < n 返回到达 n - 1 的最小跳跃次数。测试用例保证可以到达 n - 1。 示例 1: 1234输入: nums = [2,3,1,1,4]输出: 2解释: 跳到最后一个位置的最小跳跃数是 2。 从下标为 0 跳到下标为 1 的位置，跳 1 步，然后跳 3 步到达数组的最后一个位置。 示例 2: 12输入: nums = [2,3,0,1,4]输出: 2 解题思路核心思路是贪心算法，通过跟踪 “当前跳跃的最远边界” 和 “下一次跳跃的最远可达位置”，在每一步跳跃时选择最优范围，实现最少跳跃次数： 贪心策略： 用 end 表示当前跳跃能到达的最远边界（初始为 0）； 用 max_reach 表示从当前位置到 end 之间的所有位置能跳到的最远位置； 用 s...
Leetcode 0042. 接雨水
42. 接雨水给定 n 个非负整数表示每个宽度为 1 的柱子的高度图，计算按此排列的柱子，下雨之后能接多少雨水。 示例 1： 123输入：height = [0,1,0,2,1,0,1,3,2,1,2,1]输出：6解释：上面是由数组 [0,1,0,2,1,0,1,3,2,1,2,1] 表示的高度图，在这种情况下，可以接 6 个单位的雨水（蓝色部分表示雨水）。 示例 2： 12输入：height = [4,2,0,3,2,5]输出：9 解题思路核心思路是双指针边界，通过每个柱子能接住的雨水量取决于其左右两侧的最高柱子中较矮的那个（短板效应）： 边界定义： 对于位置 i，左侧最高柱子高度为 left_max[i]； 右侧最高柱子高度为 right_max[i]； 位置 i 能接住的雨水量为 min(left_max[i], right_max[i]) - height[i]（若结果为正，否则为 0）。 优化实现： 采用双指针法，无需额外存储 left_max 和 right_max 数组，将空间复杂度降至 O (1)； 用 left 和 right 指针分别从左右两端向...
Leetcode 0040. Combination Sum II
40. Combination Sum IIGiven a collection of candidate numbers (candidates) and a target number (target), find all unique combinations in candidates where the candidate numbers sum to target. Each number in candidates may only be used once in the combination. Note: The solution set must not contain duplicate combinations. Example 1: 12345678Input: candidates = [10,1,2,7,6,1,5], target = 8Output: [[1,1,6],[1,2,5],[1,7],[2,6]] Example 2: 123456Input: candidates = [2,5,2,1,2], target = 5Output:...
Leetcode 0039. Combination Sum
39. Combination SumGiven an array of distinct integers candidates and a target integer target, return a list of all unique combinations of candidates where the chosen numbers sum to target. You may return the combinations in any order. The same number may be chosen from candidates an unlimited number of times. Two combinations are unique if the frequency of at least one of the chosen numbers is different. The test cases are generated such that the number of unique combinations that sum up to ...
Leetcode 0035. Search Insert Position
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: 0 题目大意给定一个排序数组和一个目标值，在数组中找到目标值，并返回其索引。如果目标值不存在于数组中，返回它将会被按顺序插入的位置。 你可以假设数组中无重复元素。 核心思路问题本质上是要找到第一个大于等...
Leetcode 0034. Find First and Last Position of Element in Sorted Array
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 ...
Leetcode 0033. Search in Rotated Sorted Array
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 =...
Leetcode 0028. Find the Index of the First Occurrence in a String
28. Find the Index of the First Occurrence in a StringGiven two strings needle and haystack, return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack. Example 1: 1234Input: haystack = "sadbutsad", needle = "sad"Output: 0Explanation: "sad" occurs at index 0 and 6.The first occurrence is at index 0, so we return 0. Example 2: 123Input: haystack = "leetcode", needle = "leeto"Output: -1Explanation: &...
Leetcode 0027. Remove Element
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 n...
Leetcode 0026. Remove Duplicates from Sorted Array
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 num...
Leetcode 0024. Swap Nodes in Pairs
24. Swap Nodes in PairsGiven a linked list, swap every two adjacent nodes and return its head. You must solve the problem without modifying the values in the list's nodes (i.e., only nodes themselves may be changed.) Example 1: Input: head = [1,2,3,4] Output: [2,1,4,3] Explanation: Example 2: Input: head = []$ Output: [] Example 3: Input: head = [1] Output: [1] Example 4: Input: head = [1,2,3] Output: [2,1,3] 题目大意给定一个链表，要求两两交换相邻节点（如 1→2→3→4 变为 2→1→4→3），且只能通过调...
Leetcode 0020. Valid Parentheses
20. Valid ParenthesesGiven a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. An input string is valid if: Open brackets must be closed by the same type of brackets. Open brackets must be closed in the correct order. Every close bracket has a corresponding open bracket of the same type. 题目大意给定一个只包含 '('、')'、'{'、'}'、'[' 和 &...
Leetcode 0019. Remove Nth Node From End of List
19. Remove Nth Node From End of ListGiven the head of a linked list, remove the nth node from the end of the list and return its head. Example 1: 12Input: head = [1,2,3,4,5], n = 2Output: [1,2,3,5] Example 2: 12Input: head = [1], n = 1Output: [] Example 3: 12Input: head = [1,2], n = 1Output: [1] 题目大意给定一个链表的头节点，要求删除链表的倒数第 N 个节点，并返回删除后的链表头节点。 解题思路可以使用双指针法高效解决这个问题，只需一次遍历： 定义两个指针 fast 和 slow，初始都指向虚拟头节点 先让 fast 指针向前移动 N 步 然后让 fast 和 slow 同时向前移动，直到 fast 到达链表末尾 此时 slow 指针指向的就是要删除节点的前一个节点 通过调整指...
Leetcode 0017. Letter Combinations of a Phone Number
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 th...
Leetcode 0016. 3Sum Closest
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 0，在某些特殊情况下（如所有可能的和都是正数且较大）可能导致额外计算 改为使用数组前三个元素的和作为初始值，更合理地设置了起点...
Leetcode 0015. 3Sum
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 O(n^2)。这一题比较麻烦的一点在于，最后输出解的时候，要求输出不重复的解。数组中同一个数字可能出现多次，同一个数字也可能使用多次，但是最后输出解...
Leetcode 0014. Longest Common Prefix
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 <&#...
Leetcode 0013. Roman to Integer
13. Roman to Integer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 le...
Leetcode 0012. Integer to Roman
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 le...
Leetcode 0011. Container With Most Water
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 wa...
Leetcode 0010. Regular Expression Matching
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 2: 123Inpu...
Leetcode 0009. Palindrome Number
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 th...
Leetcode 0008. String to Integer (atoi)
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 p...
Leetcode 0007. Reverse Integer
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 位的有...
Leetcode 0006. Zigzag Conversion
6. Zigzag Conversion题目The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility) 123P A H NA P L S I I GY I R And then read line by line: "PAHNAPLSIIGYIR" Write the code that will take a string and make this conversion given a number of rows: 1string convert(string s, int numRows); Example 1: 12Input: s = "PAYPALISHIRING", numRows = 3Outpu...
