leetcode 287. Find the Duplicate Number (floyd判圈算法找重复元素)
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:
1. You **must not** modify the array (assume the array is read only). 2. You must use only constant, _O_(1) extra space. 3. Your runtime complexity should be less than `O(n2)`. 4. There is only one duplicate number in the array, but it could be repeated more than once. 思路：O(n^2)的复杂度暴力即可，说个O(n)复杂度的解法。