input: Write a function to count the number of words in a given string. Return the number of words as an integer.
  assert count_words('Hello world, this is a sentence.')==5
  assert count_words('This is another example.')==4
  assert count_words('Single.')==1
  assert count_words('')==0
  assert count_words('Well, this is quite a long sentence!')==7
output: def count_words(s):
      if s == '':
          return 0
      words = s.split()
      return len(words)