Given a non-empty array of integers `nums`, every element appears **twice** except for one. Find that single one.
You must implement a solution with linear runtime complexity and use only constant extra space.
**Example 1:**
```
Input: nums = [2,2,1]
Output: 1
```
**Example 2:**
```
Input: nums = [4,1,2,1,2]
Output: 4
```
**Example 3:**
```
Input: nums = [1]
Output: 1
```