input: Write a function to find the longest word in a given sentence.
  assert longestWord("This is the longest sentence.") == "sentence."
  assert longestWord("Python is great for data science.") == "science."
  assert longestWord("I love programming.") == "programming."
output: def longestWord(sentence):
      words = sentence.split()
      longest = max(words, key=len)
      return longest