input: Write a function to reverse the order of words in a given string. If the given string is empty then return an empty string.
  assert reverse_words("The quick brown fox")=="fox brown quick The"
  assert reverse_words("SingleWord")=="SingleWord"
  assert reverse_words("")==""
output: def reverse_words(s):
      words = s.split()
      return ' '.join(words[::-1])