Given an unsorted integer array, nums, return the smallest missing positive integer. Create an algorithm that runs with an $O(n)$ time complexity and utilizes a constant amount of space.

Note: The smallest missing positive isn’t the first positive number that’s missing in the range of elements in the input, but the first positive number that’s missing if we start from $1$.

Constraints:

• $1 \leq$ nums.length $\leq 10^5$

• $-2^{31} \leq$ nums[i] $\leq 2^{31} - 1$