# Time:  O(n)
# Space: O(1)

class Solution(object):
    # @param A, a list of integers
    # @return an integer
    def firstMissingPositive(self, A):
        i = 0
        while i < len(A):
            if A[i] > 0 and A[i] - 1 < len(A) and A[i] != A[A[i]-1]:
                A[A[i]-1], A[i] = A[i], A[A[i]-1]
            else:
                i += 1

        for i, integer in enumerate(A):
            if integer != i + 1:
                return i + 1
        return len(A) + 1