Missing integer

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.

Examples

• 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.

Input ranges

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].