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