find unpairedin list Problem set 1

    // You've got an array of ints 2,2,3,3,4,5,5... Find an element without a pair.
    class Program
    {
        static void Main(string[] args)
        {
            var array = new int[] {2, 2, 3, 3, 4, 5, 5};
            var array2 = new int[array.Length];

            foreach (var a in array)
            {
                if (array2[a] == 0)
                {
                    array2[a] = a;
                }
                else
                {
                    array2[a] = -1;
                }
            }

            foreach (var i in array2)
            {
                if (i > 0)
                {
                    Console.WriteLine(i);
                }
            }

            Console.ReadKey();
        }
    }