input: Write a function to reverse a given string. If the input string is empty, return an empty string as well.
  def reverse_string(input_string):  
      return input_string[::-1]
  
  assert reverse_string("Python") == "nohtyP"
  assert reverse_string("OpenAI") == "IAnepO"
  assert reverse_string("") == ""
output: def reverse_string(input_string):  
      return input_string[::-1]