challenge1.md

MissingInteger

Write a function: class Solution { public int solution(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].

Solution

private static int Solution(int[] A)
{
    int num = 1;

    if (A == null || A.Length == 0)
        return num;

    var set = new HashSet<int>(A);

    while (set.Contains(num))
    {
        num++;
    }

    return num;
}