input: Write a function to reverse the order of words in a given sentence while maintaining the order of characters in each word. If the input is an empty string, return an empty string.
  assert reverse_words('This is a sentence.')=='sentence. a is This'
  assert reverse_words('Hello World')=='World Hello'
  assert reverse_words('Python is great')=='great 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)