Solution.py

class Solution:
    def firstMissingPositive(self, nums: List[int]) -> int:
        def swap(i, j):
            nums[i], nums[j] = nums[j], nums[i]

        n = len(nums)
        for i in range(n):
            while 1 <= nums[i] <= n and nums[i] != nums[nums[i] - 1]:
                swap(i, nums[i] - 1)
        for i in range(n):
            if i + 1 != nums[i]:
                return i + 1
        return n + 1