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# Solution: Single Number

## Problem Statement

In a non-empty array of integers, every number appears twice except for one, find that single number.

**Example 1**:

```
Input: 1, 4, 2, 1, 3, 2, 3
Output: 4
```

**Example 2**:

```
Input: 7, 9, 7
Output: 9
```

**Constraints:**

- 1 <= nums.length <= 3 * 10<sup>4</sup>
- -3 * 10<sup>4</sup> <= nums[i] <= 3 * 10<sup>4</sup>
- Each element in the array appears
`twice`

except for one element which appears only once.

## Solution

One straight forward solution can be to use a **HashMap** kind of data structure and iterate through the input:

1

.....

.....

.....

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