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Given a binary tree, flatten it to a linked list in-place.For example, Given 1 / \ 2 5 / \ \ 3 4 6The flattened tree should look like: 1 \ 2

Given a binary tree and a sum, find all root-to-leaf paths where each path’s sum equals the given sum.For example: Given the below binary tree and sum = 22, 5 / \

Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.For example: Given the below binary tree and sum = 2

Given a binary tree, find its minimum depth.The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.思路：递归。这个题目相比求最大深度有些巧妙，它通过判断有没有兄弟节点来计算离根节点最近

Given a binary tree, determine if it is height-balanced.For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.For this problem, a height-balanced binary tree is defined as a binary tree in which the dep

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the tw

Given a binary tree, return the bottom-up level order traversal of its nodes’ values. (ie, from left to right, level by level from leaf to root).For example: Given binary tree [3,9,20,null,null,15,...

Given inorder and postorder traversal of a tree, construct the binary tree.Note: You may assume that duplicates do not exist in the tree.思路：这道题的思路基本和上一题一样，不同之处在于后序遍历根节点在最后面。/** * Definition for a bin

Given preorder and inorder traversal of a tree, construct the binary tree.Note: You may assume that duplicates do not exist in the tree.思路： 这道题是树中比较有难度的题目，需要根据先序遍历和中序遍历来构造出树来。这道题看似毫无头绪，其实梳理一下还是有章可循的。

Given a binary tree, find its maximum depth.The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.思路：递归。二叉树最大深度经典结论，左右分别递归下去，每层深度加1，求最大值。/**

Given a binary tree, return the zigzag level order traversal of its nodes’ values. (ie, from left to right, then right to left for the next level and alternate between).For example: Given binary tr...

Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).For example: Given binary tree [3,9,20,null,null,15,7], 3 / \ 9 20 ...

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).For example, this binary tree [1,2,2,3,4,4,3] is symmetric: 1 / \ 2 2 / \ / \3 4 4 3But the

Two elements of a binary search tree (BST) are swapped by mistake.Recover the tree without changing its structure.Note: A solution using O(n) space is pretty straight forward. Could you devise a const

Given two binary trees, write a function to check if they are the same or not.Two binary trees are considered the same if they are structurally identical and the nodes have the same value.Example 1:Inp

Given a binary tree, determine if it is a valid binary search tree (BST).Assume a BST is defined as follows:The left subtree of a node contains only nodes with keys less than the node’s key. The right

Given s1, s2, s3, find whether s3 is formed by the interleaving of s1 and s2.For example, Given: s1 = “aabcc”, s2 = “dbbca”,When s3 = “aadbbcbcac”, return true. When s3 = “aadbbbaccc”, return false

Given n, how many structurally unique BST’s (binary search trees) that store values 1…n?For example, Given n = 3, there are a total of 5 unique BST’s.1 3 3 2 1 \ /

Given an integer n, generate all structurally unique BST’s (binary search trees) that store values 1…n.For example, Given n = 3, your program should return all 5 unique BST’s shown below. 1

Given a binary tree, return the inorder traversal of its nodes’ values.For example: Given binary tree [1,null,2,3], 1 \ 2 / 3 return [1,3,2].Note: Recursive solution is t

Given a string containing only digits, restore it by returning all possible valid IP address combinations.For example: Given “25525511135”,return [“255.255.11.135”, “255.255.111.35”]. (Order does not

Reverse a linked list from position m to n. Do it in-place and in one-pass.For example: Given 1->2->3->4->5->NULL, m = 2 and n = 4,return 1->4->3->2->5->NULL.Note: Given m, n satisfy the following co

Given a collection of integers that might contain duplicates, nums, return all possible subsets (the power set).Note: The solution set must not contain duplicate subsets.For example, If nums = [1,2,2]

Given a string s1, we may represent it as a binary tree by partitioning it to two non-empty substrings recursively.Below is one possible representation of s1 = “great”:great / \ gr eat /

Given a linked list and a value x, partition it such that all nodes less than x come before nodes greater than or equal to x.You should preserve the original relative order of the nodes in each of the

Given a 2D binary matrix filled with 0’s and 1’s, find the largest rectangle containing only 1’s and return its area.For example, given the following matrix:1 0 1 0 0 1 0 1 1 1 1 1 1 1 1 1 0 0 1 0

Given n non-negative integers representing the histogram’s bar height where the width of each bar is 1, find the area of largest rectangle in the histogram.Above is a histogram where width of each bar

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.For example, Given 1->2->3->3->4->4->5, return 1-&gt...

Given a sorted linked list, delete all duplicates such that each element appear only once.For example, Given 1->1->2, return 1->2. Given 1->1->2->3->3, return 1->2->3.思路：依次遍历下一个节点，如果找到重复的节点，则用后一个取代。/

Follow up for “Search in Rotated Sorted Array”: What if duplicates are allowed?Would this affect the run-time complexity? How and why? Suppose an array sorted in ascending order is rotated at some pi

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 neig

Given a set of distinct integers, nums, return all possible subsets.Note: The solution set must not contain duplicate subsets.For example, If nums = [1,2,3], a solution is:[ [3], [1], [2],

Given two integers n and k, return all possible combinations of k numbers out of 1 … n.For example, If n = 4 and k = 2, a solution is:[ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4], ]思路

Given a string S and a string T, find the minimum window in S which will contain all the characters in T in complexity O(n).For example, S = “ADOBECODEBANC” T = “ABC” Minimum window is “BANC”.Note:

Given an array with n objects colored red, white or blue, sort them 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,

Write an efficient algorithm that searches for a value in an m x n matrix. This matrix has the following properties:Integers in each row are sorted from left to right. The first integer of each row is

Given a m x n matrix, if an element is 0, set its entire row and column to 0. Do it in place.click to show follow up.Follow up: Did you use extra space? A straight forward solution using O(mn) space

Given two words word1 and word2, find the minimum number of steps required to convert word1 to word2. (each operation is counted as 1 step.)You have the following 3 operations permitted on a word:a) In

You are climbing a stair case. It takes n steps to reach to the top.Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?Note: Given n will be a positive inte