input: Write a function to reverse the order of words in a string while keeping the order of characters in each word the same. The function should take a string as input and return the modified string.
  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)