208. 实现 Trie (前缀树)

class Trie {
private:
    vector<Trie*> children;
    bool isEnd;
public:
    /** Initialize your data structure here. */
    Trie():children(26),isEnd(false){}  //这份代码的局限性在于你得知道单词的字符范围
    
    /** Inserts a word into the trie. */
    void insert(string word) {
        Trie *nownode = this;
        for (char ch : word)
        {
            ch -= 'a'; //字符跟下标对应
            if(nownode->children[ch] == nullptr)  
                nownode->children[ch] = new Trie();
            nownode = nownode->children[ch];
        }
        nownode->isEnd = true;
    }
    
    /** Returns if the word is in the trie. */
    bool search(string word) {
        Trie * nownode = this;
        for (char ch : word)
        {
            ch -= 'a';
            if(nownode->children[ch] == nullptr)
                return false;
            nownode = nownode->children[ch];
        }
        return nownode->isEnd;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    bool startsWith(string prefix) {
        Trie *nownode = this;
        for (char ch : prefix)
        {
            ch -= 'a';
            if(nownode->children[ch] == nullptr)
                return false;
            nownode = nownode->children[ch];
        }
        return true;
    }
};