using System;
using System.Text;
using System.Collections.Generic;
using System.Diagnostics;

public class Test
{
    public static void Main()
    {
        string Text = GetText(50000);
        List<int> Indexes = GetIndexes(2000, Text.Length);
        
        Stopwatch clean1 = new Stopwatch();
        clean1.Start();
        string clean1Result = null;
        for (int i = 0; i < 100; i++)
        {
            clean1Result = Clean1(Text, Indexes);
        }
        clean1.Stop();
    
        Stopwatch clean2 = new Stopwatch();
        clean2.Start();
        string clean2Result = null;
        for (int i = 0; i < 100; i++)
        {
            clean2Result = Clean2(Text, Indexes);
        }
        clean2.Stop();
        
        Stopwatch clean3 = new Stopwatch();
        clean3.Start();
        string clean3Result = null;
        for (int i = 0; i < 100; i++)
        {
            clean3Result = Clean3(Text, Indexes);
        }
        clean3.Stop();
    
        Console.WriteLine("Clean 1: {0}", clean1.ElapsedMilliseconds);
        Console.WriteLine("Clean 2: {0}", clean2.ElapsedMilliseconds);
        Console.WriteLine("Clean 3: {0}", clean3.ElapsedMilliseconds);
    }
    
    public static string GetText(int length)
    {
        var random = new Random();
        StringBuilder sb = new StringBuilder(length);
        for (int i = 0; i < length; i++)
        {
            sb.Append(random.Next(0, 10));
        }
        return sb.ToString();
    }
    
    public static List<int> GetIndexes(int length, int textLength)
    {
        var random = new Random();
        var list = new List<int>(length);
        int max = (int)(textLength * 1.5);
        for (int i = 0; i < length; i++)
        {
            list.Add(random.Next(0, max));
        }
        return list;
    }
    
    public static string Clean1(string Text, List<int> Indexes)
    {
        HashSet<int> hashSet = new HashSet<int>(Indexes);
        StringBuilder sb = new StringBuilder(Text.Length);
        for (int i = 0; i < Text.Length; ++i)
        {
            if (!hashSet.Contains(i))
            {
                sb.Append(Text[i]);
            }
        }
        return sb.ToString();
    }

    public static string Clean2(string Text, List<int> Indexes)
    {
        const char temp = '\uFFF0';
        StringBuilder sb = new StringBuilder(Text);
        for (int i = 0; i < Indexes.Count; i++)
        {
            if (Indexes[i] < sb.Length)
            {
                sb[Indexes[i]] = temp;
            }
        }
        return sb.Replace(temp.ToString(), null).ToString();
    }

    public static string Clean3(string Text, List<int> Indexes)
    {
        bool[] exclude = new bool[Text.Length];
        for (int i = 0; i < Indexes.Count; i++)
        {
            if (Indexes[i] < exclude.Length)
            {
                exclude[Indexes[i]] = true;
            }
        }
        StringBuilder sb = new StringBuilder(Text.Length);
        for (int i = 0; i < Text.Length; i++)
        {
            if (!exclude[i])
            {
                sb.Append(Text[i]);
            }
        }
        return sb.ToString();
    }
}