input: Write a function to reverse the order of words in a given string. If the input string is empty, return an 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("")==""
output: def reverse_words(s):
      words = s.split()
      return ' '.join(words[::-1])