using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(ReverseStringInPlace("Hello World !"));

    }

    public static string ReverseStringInPlace(string str)

    {

       char[] chars = str.ToCharArray();

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

       {

        chars[i] = str[j];

        chars[j] = str[i];

       }

       return new string(chars);

    }

}