Untitled

This is a demo task.
Write a function:
int solution(vector<int> &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].
Copyright 2009-2023 by Codility Limited. All Rights Reserved. Unauthorized copying, publication or disclosure prohibited.