input: Write a python function to find the longest word in a given string.
  assert longest_word("Hello World!") == "Hello"
  assert longest_word("The quick brown fox jumped over the lazy dog") == "jumped"
  assert longest_word("A journey of a thousand miles begins with a single step") == "thousand"
output: def longest_word(s):
      words = s.split()
      longest = ""
      for word in words:
          if len(word) > len(longest):
              longest = word
      return longest