hashset

class Solution:
    def firstMissingPositive(self, nums: List[int]) -> int:
        if nums == []:
            return 1
        maxval = max(nums)
        if maxval <= 0:
            return 1
        nums = set(nums)
        for i in range(1, maxval):
            if i not in nums:
                return i
        return maxval + 1