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 take a single string as input and return the reversed sentence as a string.
  assert reverse_words('This is an example')=='example an is This'
  assert reverse_words('Hello World')=='World Hello'
  assert reverse_words('Python is fun')=='fun is Python'
  assert reverse_words('A B C D')=='D C B A'
  assert reverse_words('')==''
output: def reverse_words(sentence):
      words = sentence.split()
      reversed_sentence = ' '.join(words[::-1])
      return reversed_sentence