对于快速排序过程中一个的问题：为什么在qsort中递归时，需要将递归范围改为[lo, pos-1]而不是[lo, pos]?

Maximal Square

Given a 2D binary matrix filled with 0’s and 1’s, find the largest square containing only 1’s and return its area.

896. Monotonic Array

An array is monotonic if it is either monotone increasing or monotone decreasing.

An array A is monotone increasing if for all i <= j, A[i] <= A[j]. An array A is monotone decreasing if for all i <= j, A[i] >= A[j].

Return true if and only if the given array A is monotonic.

什么是单调栈？

287. Find the Duplicate Number

• 方法一：排序，将整个数组进行排序．排序的时间复杂度和空间复杂度取决于使用的排序算法．
  这里要求额外空间排序度为O(1)，那么只能用堆排序．
• 方法二：使用set
• 方法三
  • 前面方法一都会修改数组，不满足要求．方法二的额外空间复杂度是O(n)，同样不满足要求．
  • https://leetcode.com/problems/find-the-duplicate-number/

two sum
Two Sum II - Input array is sorted
3Sum
4Sum
4Sum II
k sum

冒泡排序的优化有设置哨兵来表示当前整个数组已经有序，以及设置有序区间，下一次只对有序区间进行排序（尚未完全理解）．

189. Rotate Array
     旋转数组

Given an array, rotate the array to the right by k steps, where k is non-negative.

题目链接

Find the sum of all left leaves in a given binary tree.

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

Note: Do not modify the linked list.