JalaSoft Test

TestJalaSoft.cs

namespace TestJalaSoft
{
    internal class Program
    {
        static void Main(string[] args)
        {
            string[] stringNumbers = new string[] { "13", "11", "9", "10", "12", "15" };
            int[] numbers = new int[stringNumbers.Length];

            for (int i = 0; i < numbers.Length; i++)
            {
                numbers[i] = Convert.ToInt32(stringNumbers[i]);
            }

            Console.WriteLine(FindLostNumber(numbers));
        }

        static int FindLostNumber(int[] numbers)
        {
            if (numbers.Length < 1)
            {
                return 0;
            }

            Array.Sort(numbers);
            int previousNumber = numbers[0];

            for (int i = 1; i < numbers.Length; i++)
            {
                if (numbers[i] != previousNumber + 1)
                {
                    return previousNumber + 1;
                }

                previousNumber = numbers[i];
            }

            return 0;
        }
    }
}