input: Write a function to reverse the order of words in a given string. If the given string is empty then return the empty string.
  assert reverse_words("Hello world")=="world Hello"
  assert reverse_words("Python programming is fun")=="fun is programming Python"
  assert reverse_words("")==""
  assert reverse_words("SingleWord")=="SingleWord"
  assert reverse_words("Two Words")=="Words Two"
output: def reverse_words(s):
      return ' '.join(s.split()[::-1])