1. 题目

设计一个方法，找出任意指定单词在一本书中的出现频率。

你的实现应该支持如下操作：

• WordsFrequency(book)构造函数，参数为字符串数组构成的一本书
• get(word)查询指定单词在数中出现的频率

示例：
WordsFrequency wordsFrequency = new WordsFrequency(
{"i", "have", "an", "apple", "he", "have", "a", "pen"});
wordsFrequency.get("you"); //返回0，"you"没有出现过
wordsFrequency.get("have"); //返回2，"have"出现2次
wordsFrequency.get("an"); //返回1
wordsFrequency.get("apple"); //返回1
wordsFrequency.get("pen"); //返回1提示：
book[i]中只包含小写字母
1 <= book.length <= 100000
1 <= book[i].length <= 10
get函数的调用次数不会超过100000

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/words-frequency-lcci
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

2. 解题

2.1 哈希解法

class WordsFrequency {unordered_map<string,int> m;
public:WordsFrequency(vector<string>& book) {for(auto& s : book)m[s]++;}int get(string word) {return m[word];}
};

2.2 Trie树

参考Trie树

class Trie
{
public:unordered_map<char,Trie*> next;bool isEnd = false;int count = 0;void insert(string& s){Trie *root = this;for(char ch : s){if(!(root->next).count(ch)){Trie* node = new Trie();root->next.insert(make_pair(ch,node));}root = root->next[ch];}root->isEnd = true;root->count++;}int search(string& s){Trie * root = this;for(char ch : s){if(!(root->next).count(ch)){return 0;}root = root->next[ch];}if(root->isEnd)return root->count;return 0;}
};
class WordsFrequency {Trie *t;
public:WordsFrequency(vector<string>& book) {t = new Trie();for(string& b : book)t->insert(b);}int get(string word) {return t->search(word);}
};