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("SingleWord")=="SingleWord"
  assert reverse_words("")==""
  assert reverse_words("   Multiple   spaces  are   here  ")=="here   are  spaces   Multiple"
output: def reverse_words(str):
      return " ".join(str.split()[::-1])