using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        char [] str = {'h', 'e', ' ' , 'l', 'l', 'o', ' ', ' ', 'w', 'o', 'r', 'l', 'd', ' ', ' ', ' ', ' ', ' ', ' '};

        urlIfy(str);

        Console.WriteLine(str);

    }

    private static void urlIfy(char[] str)

    {

        int tracker = 0;

        int tail = str.Length - 1;

        for(int i = 0; i <= str.Length - 1; i++)

        {

            if(Char.IsWhiteSpace(str[i]))

            {

                tracker = i;

                shiftLetters(tracker, tail, str);

                addLetters(tracker, str);

            }

        }

    }

    private static void shiftLetters(int currentIndex, int length, char[] str)

    {

        for (int i = length, j = length - 2; j > currentIndex; i--, j--)

        {

            str[i] = str[j];

        }

    }

    private static void addLetters(int currentIndex, char[] str)

    {

        string lettersToAdd = "%20";

        for(int i = 0; i <= lettersToAdd.Length - 1; i++)

        {

            str[i + currentIndex] = lettersToAdd[i];

        }

    }

}