input: Write a python function to compute the frequency of words in a given string.
  assert word_frequency("Python is an interpreted high-level general-purpose programming language") == {'Python': 1, 'is': 1, 'an': 1, 'interpreted': 1, 'high-level': 1, 'general-purpose': 1, 'programming': 1, 'language': 1}
  assert word_frequency("hello world hello") == {'hello': 2, 'world': 1}
  assert word_frequency("test test test") == {'test': 3}
output: def word_frequency(s):
      words = s.split()
      frequency = {}
      for word in words:
          if word in frequency:
              frequency[word] += 1
          else:
              frequency[word] = 1
      return frequency