# Detect the Only Duplicate in a List

You are given a list `nums`

of length `n + 1`

picked from the range `1, 2, ..., n`

. By the pigeonhole principle, there must be a duplicate. Find and return it. There is guaranteed to be exactly one duplicate.

Bonus: Can you do this in \(\mathcal{O}(n)$` time and `$\mathcal{O}(1)\) space?

**Constraints**

`n ≤ 10,000`

https://binarysearch.com/problems/Detect-the-Only-Duplicate-in-a-List

## Examples

### Example 1

**Input**

- nums =
`[1, 2, 3, 1]`

**Output**

- answer =
`1`

### Example 2

**Input**

- nums =
`[4, 2, 1, 3, 2]`

**Output**

- answer =
`2`

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