Simple Python Solution

  • -1

    Use an array to store whether an positive integer has appeared or not.

    class Solution(object):
        def firstMissingPositive(self, nums):
            :type nums: List[int]
            :rtype: int
            l = [0] * (len(nums) + 1)
            for i in nums:
                if i > 0 and i < len(nums)+1:
                    l[i] = 1
            for i in range(1,len(nums)+1):
                if l[i] == 0:
                    return i
            return len(nums)+1

  • 0

    You are not allowed to use an array because the space complexity has to be constant

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