using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        int[] numbers = new int[] { 1, 7, 9, 7, 4, 5, 8, 5, 4, 1, 9 };

        int? singleOccurrence = null;

        var alreadyCounted = new HashSet<int>();

​

        foreach(var item in numbers)

        {

            if (alreadyCounted.Add(item))

            {

                singleOccurrence = item;

            }

        };

​

        Console.WriteLine($"Item that only appeared once is: {singleOccurrence}");

        Console.WriteLine($"Number of items in the original collection: {numbers.Length}");

        Console.WriteLine($"Number of items in the dictionary: {alreadyCounted.Count}");

    }

}