input: Write a function to reverse the order of words in a given string. If the string is empty, return it as it is.
  assert reverse_words("Hello world")=="world Hello"
  assert reverse_words("Python is fun")=="fun is Python"
  assert reverse_words("")==""
  assert reverse_words("One")=="One"
  assert reverse_words("This is a test")=="test a is This"
output: def reverse_words(s):
      return ' '.join(s.split()[::-1]) if s else s