# Day 25 of June LeetCode Challenge

1 min readJun 25, 2020

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.

**Example 1:**

**Input:** [1,3,4,2,2]

**Output:** 2

**Example 2:**

**Input:** [3,1,3,4,2]

**Output:** 3

**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*(*n*2). - There is only one duplicate number in the array, but it could be repeated more than once.