input: Write a function to count the number of words in a given string. Return the count as an integer.
  assert count_words('Hello world, this is a sentence.')==5
  assert count_words('This is a test string with eight words.')==8
  assert count_words('One')==1
  assert count_words('')==0
  assert count_words('Two words')==2
output: def count_words(s):
      if s == '':
          return 0
      words = s.split()
      return len(words)