input: Write a function to reverse the order of words in a given string. If the input string is empty, return an empty string.
  
  def reverse_words(str):
      return " ".join(str.split()[::-1])
  
  assert reverse_words("This is a test")=="test a is This"
  assert reverse_words("Hello World")=="World Hello"
  assert reverse_words("")==""
  assert reverse_words("SingleWord")=="SingleWord"
  assert reverse_words("  Multiple spaces  ")=="spaces Multiple"
output: def reverse_words(str):
      return " ".join(str.split()[::-1])