LeetCode 287：找出重复数字的两种方法（时间复杂度分别为O(n)和O(n log n))

Find the Duplicate Number

题目连接：Find the Duplicate Number

数据描述与特征分析

Given an array nums containing n + 1 integers where each integer is between 1 and n (inclusive), prove that at least one duplicate number must exist. Assume that there is only one duplicate number, find the duplicate one.

Note:
You must not modify the array (assume the array is read only).
You must use only constant, O(1) extra space.
Your runtime complexity should be less than O(n2).
There is only one duplicate number in the arr