public class Solution

{

    public int firstMissingPositive(List<int> A)

    {

        int size = 0;

        foreach (int counter in A)

        {

            size++;

        }

        int[] arr = new int[size];

        for (int i = 0;i<size;i++)

        {

            arr[i] = -1;

        }

        for (int i = 0; i < size; i++)

        {

            if (A[i] < size && A[i] >= 1)

                arr[A[i] - 1] = 1;

        }

        for (int i = 0; i < size; i++)

        {

            if (arr[i] == -1)

                return i + 1;

        }

        return size+1;

    }

}