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