input: Write a function to reverse the order of words in a given sentence while keeping the order of characters within each word unchanged. If the input string is empty, return an empty string.
  assert reverse_words('This is a sentence.')=='sentence. a is This'
  assert reverse_words('The quick brown fox')=='fox brown quick The'
  assert reverse_words('Hello World')=='World Hello'
  assert reverse_words('')==''
  assert reverse_words('One word only')=='only word One'
output: def reverse_words(s):
      words = s.split()
      reversed_words = words[::-1]
      return ' '.join(reversed_words)