using System;

using System.Linq;

​

public static class Program

{

    public static void Main()

    {

        int[] array = new int[] { 1, 3, 6, 4, 1, 2, -1, 0 };

        Console.WriteLine($"Array: {String.Join(", ", array.AsEnumerable())}");

        int result = GetLowestPositiveNumberNotContained(array);

        Console.WriteLine($"Result: {result}");

    }

​

    public static int GetLowestPositiveNumberNotContained(int[] array)

    {

        return array

            .Where(n => n > 0)

            .Distinct()

            .Order()

            .Append(-1)

            .Zip(Enumerable.Range(1, Array.MaxLength + 1))

            .First(e => e.First != e.Second)

            .Second;

    }

}

​