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):
      if not str:
          return ""
      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("SingleWordHere")=="SingleWordHere"
  assert reverse_words("")==""
output: def reverse_words(str):
      if not str:
          return ""
      return " ".join(str.split()[::-1])