firstmissingpositive

class Solution:
    """
    @param A: An array of integers
    @return: An integer
    """

    def firstMissingPositive(self, A):
        # write your code here
        if len(A) == 0:
            return 1
        A = set(A)
        m = min(A)
        u = max(A)
        if u < 0:
            return 1
        while True:
            if m-1 not in A and m-1 > 0:
                return m -1
            if m<1:
                m += 1
            else:
                if m in A:
                    if m+1 in A:
                        m += 1
                    else:
                        return m+1
                else:
                    m += 1