input: Write a python function to find the longest word in a list of words.
  assert find_longest_word(['cat', 'elephant', 'tiger']) == 'elephant'
  assert find_longest_word(['apple', 'banana', 'kiwi']) == 'banana'
  assert find_longest_word(['dog', 'horse', 'hippopotamus']) == 'hippopotamus'
output: def find_longest_word(words):
      if not words:
          return None
      longest_word = words[0]
      for word in words[1:]:
          if len(word) > len(longest_word):
              longest_word = word
      return longest_word