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("Programming in Python")=="Python in Programming"
  assert reverse_words("One two three")=="three two One"
  assert reverse_words("")==""
output: def reverse_words(str):
      return " ".join(str.split()[::-1])