input: Write a function to reverse the order of words in a sentence but keep the order of the characters in each word the same. The function should take a string as input and return a string with the words in reverse order.
  assert reverse_words("This is a sentence.") == "sentence. a is This"
  assert reverse_words("Hello world") == "world Hello"
  assert reverse_words("Python programming") == "programming Python"
  assert reverse_words("A quick brown fox") == "fox brown quick A"
  assert reverse_words("") == ""
output: def reverse_words(s):
      words = s.split()
      reversed_words = words[::-1]
      return ' '.join(reversed_words)