input: Write a function to reverse a given string. If the input string is empty, return an empty string.
  def reverse_string(input_string):
      return input_string[::-1] if input_string else ""
  
  assert reverse_string("Python")=="nohtyP"
  assert reverse_string("")==""
  assert reverse_string("Hello World!")=="!dlroW olleH"
  assert reverse_string("12345")=="54321"
  assert reverse_string("a")=="a"
output: def reverse_string(input_string):
      return input_string[::-1] if input_string else ""