input: Write a function to reverse the order of words in a given string.
  def reverse_words(str):
      return " ".join(str.split()[::-1])
  
  assert reverse_words("hello world")=="world hello"
  assert reverse_words("The quick brown fox")=="fox brown quick The"
  assert reverse_words("")==""
  assert reverse_words("Hello")=="Hello"
  assert reverse_words("Python is fun")=="fun is Python"
output: def reverse_words(str):
      return " ".join(str.split()[::-1])