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Given an array of n integers where n > 1, nums, return an array output such that output[i] is equal to the product of all the elements of nums except nums[i].
Solve it without division and in O(n).
For example, given [1,2,3,4], return [24,12,8,6].
我们创建一个长度和原数组num[]长度一样的数组result[]，先从左边开始，让result ...
Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.
According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a des ...
Given a singly linked list, determine if it is a palindrome.
Follow up:
Could you do it in O(n) time and O(1) space?
判断一个单向链表是否为回文链表，要求时间复杂度为O(n)，空间复杂度为O(1)。我们可以先用快慢指针找到中间的节点，然后将后半部分链表反转，然后以前半部分的链表进行比较，如果相同就返回true，如果不同就返回false。代码如下：
/**
* Definition for singly-linked list.
* public class List ...
Implement the following operations of a queue using stacks.
push(x) -- Push element x to the back of queue.
pop() -- Removes the element from in front of queue.
peek() -- Get the front element.
empty() -- Return whether the queue is empty.
Notes:
You must use only standard operations of a stack -- w ...
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.
给定一个二叉搜索树，从中找到第k小的元素。根据二叉搜索树的性质中序遍历正好是二叉搜索树的升序排列，当我们遍历到第k个元素是返回就可以了。代码如下：
/**
* Definition for a binary tree node.
* public c ...
Given an integer array of size n, find all elements that appear more than ⌊ n/3 ⌋ times. The algorithm should run in linear time and in O(1) space.
这道题目是Majority Element的变形，这里要求找出出现次数多余n/3次。我们通过分析可以知道这样的元素可能有两个，我们同样可以采用Moore算法，与上一题目不同的的是三个元素一组，这样在线性的时间复杂度下可以解决。代码如下：
public class Solution {
pu ...
Given a sorted integer array without duplicates, return the summary of its ranges.
For example, given [0,1,2,4,5,7], return ["0->2","4->5","7"].
从第一个元素开始，每次保留开始的元素的值，依次扫描后面的元素，如果有连续的就一直往后查找，直到遇到不连续的，然后记录这段范围。直到遍历完所有的元素。代码如下：
public class Solution {
public ...
Invert a binary tree.
4
/ \
2 7
/ \ / \
1 3 6 9
to
4
/ \
7 2
/ \ / \
9 6 3 1
给定一个二叉树，交换它的左右子树。我们用递归完成，从根节点开始，递归交换root的左子树，然后递归交换root的右子树。代码如下：
/**
* Definition for a binary tree node.
* public class TreeNode {
* int va ...
Implement the following operations of a stack using queues.
push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
empty() -- Return whether the stack is empty.
Notes:
You must use only standard operations of a queue -- which means only ...
Find the total area covered by two rectilinear rectangles in a 2D plane.
Each rectangle is defined by its bottom left corner and top right corner as shown in the figure.
Assume that the total area is never beyond the maximum possible value of int.
首先计算出两个长方形的面积，然后计算出它们重叠的面积，最后返回两个面积的差就可以了。代码如下：
p ...
Given a complete binary tree, count the number of nodes.
Definition of a complete binary tree from Wikipedia:
In a complete binary tree every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes inclusi ...
Given a 2D binary matrix filled with 0's and 1's, find the largest square containing all 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
Return 4.
典型的二维动态规划的问题。我们构造一个二维的dp数组，dp[i][j]代表到点(i , j)时由1构成的正方形的最大长度。当char[i][j] 为0时，dp[i][j]在当前点只能为0 ...
Given an array of integers, find out whether there are two distinct indices i and j in the array such that the difference between nums[i] and nums[j] is at most t and the difference between i and j is at most k.
题目中给定一个数组，判断数组中是否存在两个数nums[i]和nums[j]，它们之间的差最大为t, 两个下标i和j之间的差最大为k。我们用TreeSet来解决，我们维护一个大小 ...
Given an array of integers and an integer k, find out whether there are two distinct indices i and j in the array such that nums[i] = nums[j] and the difference between i and j is at most k.
给定一个数组判断数组中是否存在两个相同的数，它们之间的差最大为k。我们借助HashMap, key中存放数组的元素nums[i]，value中存放对应的下标i。如果遇到相同的key，就对比当前元素的下标i与key中va ...
Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.
Ensure that numbers within the set are sorted in ascending order.
Example 1:
Input: k = 3, n = 7
Output:
[[1,2,4]]
Exam ...