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("Python programming is fun")=="fun is programming Python"
  assert reverse_words("This is a test")=="test a is This"
  assert reverse_words("")==""
  assert reverse_words("single")=="single"
output: def reverse_words(str):
      return " ".join(str.split()[::-1])