message: update longest-word-in-dictionary.py
  a bit more pythonic way
diff: # Time: O(n), n is the total sum of the lengths of words
  # Space: O(t), t is the number of nodes in trie
  -import collections
  +from collections import defaultdict
  +from operator import getitem
  class Solution(object):
  @@ -10,10 +11,10 @@ class Solution(object):
  :type words: List[str]
  :rtype: str
  """
  - _trie = lambda: collections.defaultdict(_trie)
  + _trie = lambda: defaultdict(_trie)
  trie = _trie()
  for i, word in enumerate(words):
  - reduce(dict.__getitem__, word, trie)["_end"] = i
  + reduce(getitem, word, trie)["_end"] = i
  # DFS
  stack = trie.values()
  