easy

ProblemGiven two strings s and t, determine if they are isomorphic. Two strings s and t are isomorphic if the characters in s can be replaced to get t. All occurrences of a character must be replaced with another character while preserving the order of characters. No two characters may map to the same character, but a character may map to itself. Example 1: Input: s = “egg”, t = “add” Output: true Explanation: The strings s and t can be made identical by: Mapping 'e' to 'a ...

ProblemGiven two strings ransomNote and magazine, return true if ransomNote can be constructed by using the letters from magazine and false otherwise. Each letter in magazine can only be used once in ransomNote. Example 1: 123Input: ransomNote = "a", magazine = "b"Output: false Example 2: 12Input: ransomNote = "aa", magazine = "ab"Output: false Example 3: 123Input: ransomNote = "aa", magazine = "aab"Output: true Constraints: 1 <= ...

ProblemGiven 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: 123Input: s = "()"Output: true Example 2: 123Input: s = "()[]{}&q ...

ProblemGiven the root of a binary tree, check whether it is a mirror of itself (i.e., symmetric around its center). Example 1: !https://assets.leetcode.com/uploads/2021/02/19/symtree1.jpg 123Input: root = [1,2,2,3,4,4,3]Output: true Example 2: !https://assets.leetcode.com/uploads/2021/02/19/symtree2.jpg 123Input: root = [1,2,2,null,3,null,3]Output: false Constraints: The number of nodes in the tree is in the range [1, 1000]. 100 <= Node.val <= 100 Follow up: Could you solve it both r ...

ProblemGiven the root of a binary tree, return its maximum depth. A binary tree’s maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node. Example 1: !https://assets.leetcode.com/uploads/2020/11/26/tmp-tree.jpg 123Input: root = [3,9,20,null,null,15,7]Output: 3 Example 2: 123Input: root = [1,null,2]Output: 2 Constraints: The number of nodes in the tree is in the range [0, 104]. 100 <= Node.val <= 100 Solve解11234567891011121314151617 ...

ProblemGiven an array nums of size n, return the majority element. The majority element is the element that appears more than ⌊n / 2⌋ times. You may assume that the majority element always exists in the array. Example 1: 123Input: nums = [3,2,3]Output: 3 Example 2: 123Input: nums = [2,2,1,1,1,2,2]Output: 2 Constraints: n == nums.length 1 <= n <= 5 * 104 109 <= nums[i] <= 109 Follow-up: Could you solve the problem in linear time and in 1O(1) space? Solve解112345678910class Solutio ...

ProblemThere are n teams numbered from 0 to n - 1 in a tournament. Given a 0-indexed 2D boolean matrix grid of size n * n. For all i, j that 0 <= i, j <= n - 1 and i != j team i is stronger than team j if grid[i][j] == 1, otherwise, team j is stronger than team i. Team a will be the champion of the tournament if there is no team b that is stronger than team a. Return the team that will be the champion of the tournament. Example 1: 12345Input: grid = [[0,1],[0,0]]Output: 0Explanation: There ...

ProblemYou have a RecentCounter class which counts the number of recent requests within a certain time frame. Implement the RecentCounter class: RecentCounter() Initializes the counter with zero recent requests. int ping(int t) Adds a new request at time t, where t represents some time in milliseconds, and returns the number of requests that has happened in the past 3000 milliseconds (including the new request). Specifically, return the number of requests that have happened in the inclusive ran ...

ProblemGiven an integer array nums sorted in non-decreasing order, remove the duplicates in-place such that each unique element appears only once. The relative order of the elements should be kept the same. Then return the number of unique elements in nums. Consider the number of unique elements of nums to be k, to get accepted, you need to do the following things: Change the array nums such that the first k elements of nums contain the unique elements in the order they were present in nums ini ...

ProblemGiven two strings s and t, return true if s is a subsequence of t, or false otherwise. A subsequence of a string is a new string that is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (i.e., "ace" is a subsequence of "abcde" while "aec" is not). Example 1: 123Input: s = "abc", t = "ahbgdc"Output: true Example 2: 123Input: s = "axc ...