public void Reversal(string input)
  {
   string result = "";
   for (int i = input.Length - 1; i >= 0; i--)
   {
    result += input[i];
   }
   Console.WriteLine(result);
  }

public void Reversal1(string input)
  {
   char[] array = input.ToCharArray();
   IEnumerable<char> cs = array.Reverse<char>();
   char[] array1 = cs.ToArray<char>();
   string result = new string(array1);
   Console.WriteLine(result);
  }

public void Reversal2(string input)
  {
   string result = new string(input.ToCharArray().Reverse<char>().ToArray<char>());
   Console.WriteLine(result);
  }

public void Reversal3(string input)
  {
   char[] array = input.ToCharArray();
   Array.Reverse(array);
   string result = new string(array);
   Console.WriteLine(result);
  }
 

public static string Reverse(this string input)
  {
   // return new string(input.ToCharArray().Reverse().ToArray());
   return new string(input.ToCharArray().Reverse<char>().ToArray<char>());
  }