input: Write a python function to find the longest word in a sentence.
  assert longestWord("The quick brown fox jumps over the lazy dog") == "jumps"
  assert longestWord("A journey of a thousand miles begins with a single step") == "thousand"
  assert longestWord("To be or not to be, that is the question") == "question"
output: def longestWord(sentence):
      words = sentence.split()
      longest = max(words, key=len)
      return longest