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 given sentence is empty, return an empty string.
  
  assert reverse_words('This is a sentence.')=='sentence. a is This'
  assert reverse_words('Python is fun')=='fun is Python'
  assert reverse_words('Hello World')=='World Hello'
  assert reverse_words('One two three')=='three two One'
  assert reverse_words('')==''
output: def reverse_words(sentence):
      words = sentence.split()
      reversed_words = words[::-1]
      return ' '.join(reversed_words)