题目链接：720. 词典中最长的单词
题目比较难以理解，合法的单词，它的前缀也必须在词典中。在所有合法的单词中，寻找最长的那个。暴力解法。
from typing import List
class Solution:
def longestWord(self, words: List[str]) -> str:
words = set(words)
for word in sorted(words, key=lambda w: (-len(w), w)):
for i in range(1, len(word)):
if word[:i] not in words:
break
else:
return word
return ''