input: Write a python function to reverse the words in a given string.
  assert reverse_words('hello world') == 'world hello'
  assert reverse_words('the quick brown fox') == 'fox brown quick the'
  assert reverse_words('a b c d e f g') == 'g f e d c b a'
output: def reverse_words(s):
      return ' '.join(s.split()[::-1])