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 constant extra space.
Example 1:
Input: nums = [1,2,0] Output: 3
Example 2:
Input: nums = [3,4,-1,1] Output: 2
Example 3:
Input: nums = [7,8,9,11,12] Output: 1
Constraints:
1 <= nums.length <= 5 * 105-231 <= nums[i] <= 231 - 1