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### leetcode Find the Duplicate Number

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.

• 你不能修改这个数组
• 空间复杂度要求O(1)
• 时间复杂度小于O(n2)
• 只有一个元素是重复的，但可能重复不止一次

• 0 - > 2
• 1  -> 4
• 2  -> 1
• 3  -> 3

• 0 - > 2
• 1  -> 4
• 2  -> 1
• 3  -> 3
• 4  ->3