Leetcode

20. Valid Parentheses Hardness: \(\color{green}\textsf{Easy}\) Ralated Topics: String、Stack 一、題目 Given 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. Example 1: ...

1706. Where Will the Ball Fall Hardness: \(\color{orange}\textsf{Medium}\) Ralated Topics: Array、Dynamic Programming、Depth-First Search、Matrix、Simulation 一、題目 You have a 2-D grid of size m x n representing a box, and you have n balls. The box is open on the top and bottom sides. Each cell in the box has a diagonal board spanning two corners of the cell that can redirect a ball to the right or to the left. A board that redirects the ball to the right spans the top-left corner to the bottom-right corner and is represented in the grid as 1. A board that redirects the ball to the left spans the top-right corner to the bottom-left corner and is represented in the grid as -1. We drop one ball at the top of each column of the box. Each ball can get stuck in the box or fall out of the bottom. A ball get stuck if it hits a “V” shaped pattern between two boards or if a board redirects the ball into either wall of the box. Return an array answer of size n where answer[i] is the column that the ball falls out of at the bottom after dropping the ball from the ith column at the top, or -1 if the ball gets stuck in the box. Example 1: ...

766. Toeplitz Matrix Hardness: \(\color{green}\textsf{Easy}\) Ralated Topics: Array、Matrix 一、題目 Given an m x n matrix, return true if the matrix is Toeplitz. Otherwise, return false. A matrix is Toeplitz if every diagonal from top-left to bottom-right has the same elements. Example 1: Input: matrix = [[1,2,3,4],[5,1,2,3],[9,5,1,2]] Output: true Explanation: In the above grid, the diagonals are: “[9]”, “[5, 5]”, “[1, 1, 1]”, “[2, 2, 2]”, “[3, 3]”, “[4]”. In each diagonal all elements are the same, so the answer is True. Example 2: ...

19. Remove Nth Node From End of List Hardness: \(\color{orange}\textsf{Medium}\) Ralated Topics: Linked List、Two Pointers 一、題目 Given the head of a linked list, remove the nth node from the end of the list and return its head. Example 1: Input: head = [1,2,3,4,5], n = 2 Output: [1,2,3,5] Example 2: Input: head = [1], n = 1 Output: [] Example 3: Input: head = [1,2], n = 1 Output: [1] Constraints: ...

1293. Shortest Path in a Grid with Obstacles Elimination Hardness: \(\color{red}\textsf{Hard}\) Ralated Topics: Array、Breadth-First Search、Matrix 一、題目 You are given an m x n integer matrix grid where each cell is either 0 (empty) or 1 (obstacles). You can move up, down, left, or right from and to an empty cell in one step. Return the minimum number of steps to walk from the upper left corner (0, 0) to the lower right conrer m-1, n-1 given that you can eliminate at most k obstacles. If it is not possible to find such walk return -1. ...

2136. Earliest Possible Day of Full Bloom Hardness: \(\color{red}\textsf{Hard}\) Ralated Topics: Array、Greedy、Sorting 一、題目 You have n flower seeds. Every seed must be planted first before it can begin to grow, then bloom. Planting a seed take time and so does the growth of a seed. You are given two 0-indexed integer arrays plantTime and growTime, of length n each: plantTime[i] is the number of full days it takes you to plant the ith seed. Every day, you can work on planting exactly one seed. You do not have to work on planting the same seed on consecutive days, but the planting of a seed is not complete until you have worked plantTime[i] days on planting it in total. growTime[i] is the number of full days it takes the ith seed to grow after being completely planted. After the last day of its growth, the flower blooms and stays bloomed forever. From the beginning of day 0, you can plant the seeds in any order. Return the earliest possible day where all seeds are blooming. Example 1: ...

18. 4Sum Hardness: \(\color{orange}\textsf{Medium}\) Ralated Topics: Array、Two Pointer、Sorting 一、題目 Given an array nums of n integers, return an array of all the unique quadruplets [nums[a], nums[b], nums[c], nums[d]] such that: 0 <= a, b, c, d < n a, b, c and d are distinct. nums[a] + nums[b] + nums[c] + nums[d] == target You may return the answer in any order. Example 1: Input: nums = [1,0,-1,0,-2,2], target = 0 Output: [[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]] Example 2: ...

17. Letter Combinations of a Phone Number Hardness: \(\color{orange}\textsf{Medium}\) Ralated Topics: Hash Table,String,Backtracking 一、題目 Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters. Example 1: Input: digits = “23” Output: [“ad”,“ae”,“af”,“bd”,“be”,“bf”,“cd”,“ce”,“cf”] Example 2: ...

no. Hardness: \(\color{orange}\textsf{Medium}\) Ralated Topics: Array、Two Pointers、Sorting 一、題目 Given an integer array nums of length n and an integer target, find three integers in nums such that the sum is closet to target. Return *the sum of the three integers`. You may assume that each input would have exactly one solution. Example 1: Input: nums = [-1,2,1,-4], target = 1 Output: 2 Explanation: The sum that is closet to the target is 2. (-1 + 2 + 1 = 2). Example 2: ...

15. 3Sum Hardness: \(\color{orange}\textsf{Medium}\) Ralated Topics: Array、Two Pointer、Sorting 一、題目 Given an integer array nums, return all the triplets [nums[i], nums[j], nums[k]] such that i != j, i != k, j != k, and nums[i] + nums[j] + nums[k] == 0. Notice that the solution set must not contain duplicate triplets. Example 1: Input: nums = [-1,0,1,2,-1,-4] Output: [[-1,-1,2],[-1,0,1]] Explanation: nums[0] + nums[1] + nums[2] = (-1) + 0 + 1 = 0. nums[1] + nums[2] + nums[4] = 0 + 1 + (-1) = 0. nums[0] + nums[3] + nums[4] = (-1) + 2 + (-1) = 0. The distinct triplets are [-1,0,1] and [-1,-1,2]. Notice that the order of the output and the order of the triplets does not matter. Example 2: ...