input: Write a function to reverse the order of words in a given sentence while keeping the order of characters in each word unchanged. The function should accept 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 is fun') == 'fun is Python'
  assert reverse_words('') == ''
  assert reverse_words('SingleWord') == 'SingleWord'
output: def reverse_words(s):
      words = s.split()
      reversed_words = words[::-1]
      return ' '.join(reversed_words)