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

Given an unsorted integer array nums. Return the smallest positive integer that is not present in nums.
You must implement an algorithm that runs in O(n) time and uses O(1) auxiliary space.
 
Please complete the following python code precisely:
```python
class Solution:
    def firstMissingPositive(self, nums: List[int]) -> int:
```