input: Write a function to reverse the order of words in a given sentence while maintaining the order of characters in each word.
  
  def reverse_words(string):
    words = string.split(" ")
    reversed_words = []
    for word in words:
      reversed_words.insert(0, word)
    return ' '.join(reversed_words)
  
  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(string):
    words = string.split(" ")
    reversed_words = words[::-1]
    return ' '.join(reversed_words)