Given an unsorted list of integers, find the smallest missing positive integer.
Your algorithm should run in O(n) time and use constant extra space.
Examples
Example 1:
Input: nums = [1, 2, 0]
Output: 3
Explanation: The numbers 1 and 2 are present, so the first missing positive is 3.
Example 2:
Input: nums = [3, 4, -1, 1]
Output: 2
Explanation: 1 is present but 2 is missing, so the answer is 2.
Example 3:
Input: nums = [7, 8, 9, 11, 12]
Output: 1
Explanation: No positive integers starting from 1 are present, so the answer is 1.
Example 4:
Input: nums = [1, 2, 3, 4, 5]
Output: 6
Explanation: All numbers from 1 to 5 are present, so the first missing positive is 6.
Example 5:
Input: nums = []
Output: 1
Explanation: An empty list has no positive integers, so the answer is 1.
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