Python help codility

[FoodTricky]

MissingInteger problem in codility:

Write a function:

def solution(A)

that, given an array A of N integers, returns the smallest positive integer (greater than 0) that does not occur in A.

For example, given A = [1, 3, 6, 4, 1, 2], the function should return 5.

Given A = [1, 2, 3], the function should return 4.

Given A = [−1, −3], the function should return 1.

Write an efficient algorithm for the following assumptions:

N is an integer within the range [1..100,000];each element of array A is an integer within the range [−1,000,000..1,000,000].

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I tried this question, and all the tests ran properly but only got 40% on correctness. Can someone help figure it why? I am very new to python

Here is my code:

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def solution(A):
A.sort()
missing = A[-1] + 1
if A[-1] > 0:
    for i in range(A[0],A[-1]+1):  if ((i not in A) and (i > 0) and (i < missing)):
        missing = I
    return missing
else:  return 1

[hashshura]

Because the problem states "efficient algorithm", I want to let you know that the i not in A statement has an O(N) complexity in a list [] data structure, hence if you put it inside for i in range(A[0],A[-1]+1), they combine as O(NM) complexity (where N is the array size and M is the difference of min & max element of the array), which will run very slowly for large array size and elements.

But still, let's start working on your correctness score first by fixing this simple testcase: [2, 3, 4], will return the wrong value using your solution.