input: Write a function to reverse the order of words in a given string. If the given string is empty then return the original string.
  assert reverse_words("Hello world")=="world Hello"
  assert reverse_words("Python is awesome")=="awesome is Python"
  assert reverse_words("The quick brown fox")=="fox brown quick The"
  assert reverse_words("")==""
output: def reverse_words(s):
      if not s:
          return s
      words = s.split()
      return ' '.join(words[::-1])