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 is awesome")=="awesome is Python"
  assert reverse_words("Reverse these words")=="words these Reverse"
  assert reverse_words("")==""
  assert reverse_words("One")=="One"
output: def reverse_words(str):
      return " ".join(str.split()[::-1])