Given an unsorted integer array nums, return the smallest missing positive integer.

You must implement an algorithm that runs in O(n) time and uses O(1) auxiliary space.

  Example 1:

Input: nums = [1,2,0] Output: 3 Explanation: The numbers in the range [1,2] are all in the array.

Example 2:

Input: nums = [3,4,-1,1] Output: 2 Explanation: 1 is in the array but 2 is missing.

Example 3:

Input: nums = [7,8,9,11,12] Output: 1 Explanation: The smallest positive integer 1 is missing.

  Constraints:

1 <= nums.length <= 10⁵
-2³¹ <= nums[i] <= 2³¹ - 1