Please solve the programming task below using a self-contained code snippet in a markdown code block.

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 a linear runtime complexity and use only constant extra space.
 
Please complete the following python code precisely:
```python
class Solution:
    def singleNumber(self, nums: List[int]) -> int:
```