# single-number

Created by [@theoludwig](https://github.com/theoludwig) on 21 August 2023.

## Instructions

Given a **non-empty** array of integers, every element appears twice except for one. Find that single one.

You must implement a solution with a linear runtime complexity and use only constant extra space.

### Constraints

- $$1 <= numbers.length <= 3 * 10^4$$
- $$-3 * 10^4 <= numbers[index] <= 3 * 10^4$$
- Each element in the array appears twice except for one element which appears only once.

## Source

[LeetCode - Single Number](https://leetcode.com/problems/single-number/)

## Examples

See the `test` folder for examples of input/output.

### Example 1

#### Input

```txt
4
1
2
1
2
```

#### Output

```txt
4
```