https://github.com/shadowdsp/LeetCode/tree/master/2018%E7%A7%8B%E6%8B%9B%E7%BD%91%E6%98%93%E6%B8%B8%E6%88%8F%E6%8F%90%E5%89%8D%E6%89%B9 前三题模拟，第四题字典树。时间比较赶，写的很丑。