input: Write a function to count the frequency of each word in a given string.
  assert word_count("This is a test string to test the word count program") == {'This': 1, 'is': 1, 'a': 1, 'test': 2, 'string': 1, 'to': 1, 'the': 1, 'word': 1, 'count': 1, 'program': 1}
  assert word_count("hello world hello") == {'hello': 2, 'world': 1}
  assert word_count("") == {}
output: def word_count(s):
      words = s.split()
      frequency = {}
      for word in words:
          frequency[word] = frequency.get(word, 0) + 1
      return frequency