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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...

105. Construct Binary Tree from Preorder and Inorder Traversal 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 # Definition for a binary tree node. # class TreeNode(object): # def __init..

103. Binary Tree Zigzag Level Order Traversal Given a binary tree, return the zigzag level ordertraversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between). For example: Given binary tree [3,9,20,null,null,15,7], 3 / \ 9 20 / \ 15 7 return its zigzag level order traversal as: [ [3], [20,9], [15,7] ] class Solution(object): def zigzagLevel..

94. Binary Tree Inorder Traversal Given a binary tree, return the inorder traversal of its nodes' values. Example: Input: [1,null,2,3] 1 \ 2 / 3 Output: [1,3,2] class Solution(object): def inorderTraversal(self, root): """ :type root: TreeNode :rtype: List[int] """ if not root: return [] else: return self.inorderTraversal(root.left) + [root.val] + self.inorderTraversal(root.right)

160. Intersection of Two Linked Lists Write a program to find the node at which the intersection of two singly linked lists begins. For example, the following two linked lists: begin to intersect at node c1. Example 1: Input: intersectVal = 8, listA = [4,1,8,4,5], listB = [5,0,1,8,4,5], skipA = 2, skipB = 3 Output: Reference of the node with value = 8 Input Explanation: The intersected node's va..

2. Add Two Numbers 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 contain a single digit. Add the two numbers and return it as a linked list. You may assume the two numbers do not contain any leading zero, except the number 0 itself. Example: Input: (2 -> 4 -> 3) + (5 -> 6 -> 4) Output: 7 -> 0 -> 8 E..

문제 정수 배열에서 peak란 현재 원소가 인접한 정수보다 크거나 같을 때를 말하고, 'valley'란 현재 원소가 인접한 정수보다 작거나 같을 때를 말한다. 예를 들어 {5,8,6,2,3,4,6}이 있으면, {8,6}은 peak고 {5,2}는 valley인 것이다. 정수배열이 주어졌을때 'peak'와 'valley'가 번갈아 등장하도록 정렬하는 알고리즘을 작성하라. 해법 1. 부분 최적 해법 배열을 정렬하고 위치를 바꿈으로써 peak와 valley를 순차적으로 나올 수 있도록 만들 수 있을 것이다. 정렬된 배열 0 1 4 7 8 9 가 있다고 하자. 0은 올바른 위치고 1은 잘못 놓였다 0과 4중에 자리를 바꿀 수 있으나 0와 바꿔보자 7도 잘못 놓였다. 4과 8중에 자리를 바꿀 수 있으나 4과 바꿔보..