input: Write a function to reverse the order of words in a given string. If the input string is empty, return an empty string.
  assert reverse_words("hello world")=="world hello"
  assert reverse_words("Python programming is fun")=="fun is programming Python"
  assert reverse_words("This is a test")=="test a is This"
  assert reverse_words("")==""
output: def reverse_words(s):
      if not s:
          return ""
      words = s.split()
      return " ".join(words[::-1])