using System;

​

public static class Program

{

    public static void Main()

    {

        var array = new[] { 1, 2, 3, 4, 5, 501, 993 };

        Console.WriteLine("Minimum not in list: {0}", array.Smallest());

    }

    public static int Smallest(this int[] items) {

        int startIndex = 0, i = 0, result = items[0];

        while (i < items.Length) {

            if (items[i] == result) {

                result++;

                i = startIndex;

            } else {

                if (items[i] < result) {

                    if (i != startIndex) {

                        int temp = items[startIndex];

                        items[startIndex] = items[i];

                        items[i] = temp;

                    }

                    startIndex++;

                }

                i++;

            }

        }

        return result;

    }

}