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347. Top K Frequent Elements Given a non-empty array of integers, return the kmost frequent elements. Example 1: Input: nums = [1,1,1,2,2,3], k = 2 Output: [1,2] Example 2: Input: nums = [1], k = 1 Output: [1] Note: You may assume k is always valid, 1 ≤ k ≤ number of unique elements. Your algorithm's time complexity must bebetter than O(n log n), where n is the array's size. class Solution(objec..

75. Sort Colors Given an array with n objects colored red, white or blue, sort them in-place so that objects of the same color are adjacent, with the colors in the order red, white and blue. Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively. Note: You are not suppose to use the library's sort function for this problem. Example: Input: [2,0,2,1,1,..

79. Word Search Given a 2D board and a word, find if the word exists in the grid. The word can be constructed from letters of sequentially adjacent cell, where "adjacent" cells are those horizontally or vertically neighboring. The same letter cell may not be used more than once. Example: board = [ ['A','B','C','E'], ['S','F','C','S'], ['A','D','E','E'] ] Given word = "ABCCED", return true. Given..

78. Subsets Given a set of distinct integers, nums, return all possible subsets (the power set). Note: The solution set must not contain duplicate subsets. Example: Input: nums = [1,2,3] Output: [ [3], [1], [2], [1,2,3], [1,3], [2,3], [1,2], [] ] class Solution(object): def subsets(self, nums): index = 0 curs = [[]] len_nums = len(nums) while index < len_nums: next_step =[] for cur in curs: next..

46. Permutations Given a collection of distinct integers, return all possible permutations. Example: Input: [1,2,3] Output: [ [1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], [3,2,1] ] class Solution(object): def Permute(self, cur ,nums, len_nums, result): if not nums: result.append([i for i in cur]) return for i in range(len(nums)) : cur.append(nums[i]) self.Permute(cur, nums[:i]+nums[i+1:], len_nu..

22. Generate Parentheses Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses. For example, given n = 3, a solution set is: [ "((()))", "(()())", "(())()", "()(())", "()()()" ] class Solution(object): def makeCombi(self, prefix, n , openp, closep, result): #print(prefix) if openp < closep : return if openp+ closep == 2*n : result.append(prefix) i..

17. Letter Combinations of a Phone Number 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: Input: "23" Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"]. Note: Although the ab..

200. Number of Islands Given a 2d grid map of '1's (land) and '0's (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water. Example 1: Input: 11110 11010 11000 00000 Output: 1 Example 2: Input: 11000 11000 00100 00011 Output: 3 BFS 또는 DFS로 풀면..

230. Kth Smallest Element in a BST Given a binary search tree, write a function kthSmallest to find the kth smallest element in it. Note: You may assume k is always valid, 1 ≤ k ≤ BST's total elements. Example 1: Input: root = [3,1,4,null,2] k = 1 3 / \ 1 4 \ 2 Output: 1 Example 2: Input: root = [5,3,6,2,4,null,null,1], k = 3 5 / \ 3 6 / \ 2 4 / 1 Output: 3 Follow up: What if the BST is modified..

116. Populating Next Right Pointers in Each Node You are given a perfect binary tree where all leaves are on the same level, and every parent has two children. The binary tree has the following definition: struct Node{ int val; Node *left; Node *right; Node *next; } Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to NULL...