using System;

using System.Collections.Generic;

public class Program

{

    private static HashSet<ushort> indexSet;

    public static void Main()

    {

        indexSet = new HashSet<ushort>();

        var timer = new System.Diagnostics.Stopwatch();

        timer.Start();

        ushort index = 0;

        while(index < 50000){

            indexSet.Add(index++);

        }

        Console.WriteLine(string.Format("{0}", timer.ElapsedMilliseconds / 1000f));

        timer.Reset();

        timer.Start();

        Console.WriteLine(string.Format("{0} {1}", FindNext(), timer.ElapsedMilliseconds / 1000f));

        var cycles = 1000;

        timer.Reset();

        timer.Start();

        while(cycles-- > 0){ 

            indexSet.Remove(4523);

            FindNext();

            indexSet.Remove(46312);

            FindNext();

            indexSet.Remove(12352);

            FindNext();

        }

        Console.WriteLine(string.Format("{0} {1}", FindNext(), timer.ElapsedMilliseconds / 1000f));

    }

    private static ushort FindNext() {

        ushort index = 0;

        while(!indexSet.Add(++index)) {     

        }

        return index;

    }

}