input: from typing import List
  
  def reverse_words(sentence: str) -> str:
      """ Reverses the order of words in a given sentence while keeping the order of characters in each word unchanged.
      >>> reverse_words('This is a sentence.')
      'sentence. a is This'
      >>> reverse_words('Hello world')
      'world Hello'
      """
      words = sentence.split()
      reversed_sentence = ' '.join(words[::-1])
      return reversed_sentence
output:     words = sentence.split()
      reversed_sentence = ' '.join(words[::-1])
      return reversed_sentence