using System;

using System.Collections;

public class Program

{

    public static void Main()

    {

        string str = "Hello World!";

        Console.WriteLine(reverseString(str));

        var k = new ArrayList();

        k.Add('k');

        k.Add(0);

        var lo = new int[0];

        //lo[0] = 1;

        //lo[1] = 2;

        //lo[2] = 'o';

    }

    private static string reverseString(string str){

        Char[] chars = str.ToCharArray();

        int j = chars.Length-1;

        for(int i = 0; i < chars.Length/2; i++){

            char temp = chars[i];

            chars[i] = chars[j];

            chars[j] = temp;

            j--;

        }

        return new String(chars);   

    }

​

}