# C# solution for Leetcode question: Find the Duplicate Number (difficulty : easy)

Source:

https://leetcode.com/problems/find-the-duplicate-number/#/description

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 array, but it could be repeated more than once.

My C# solution is here: