M

代码说明：

  字典树的基本功能是用来查询某个单词（前缀）在所有单词中出现次数的一种数据结构，它的插入和查询复杂度都为O(len),Len为单词（前缀）长度，但是它的空间复杂度却非常高，如果字符集是26个字母，那每个节点的度就有26个，典型的以空间换时间结构。(The basic function of the dictionary tree data structure is used to query a number of times a word (prefix) appears in all the words, which insertion and query complexity are O (len), Len is a word (prefix) length, but its space complexity is very high, if the character set is 26 letters, and that the degree of each node will have 26, the typical structure of space for time.)