题意：实现Trie

class TrieNode{
public:
    bool isword;
    TrieNode* nex[26];
    TrieNode():isword(false){
        for(int i = 0; i < 26; ++i) nex[i] = NULL;
    }
};
class Trie {
public:
    TrieNode *root;
    /** Initialize your data structure here. */
    Trie() {
        root = new TrieNode();
    }
    /** Inserts a word into the trie. */
    void insert(string word) {
        TrieNode *p = root;
        int len = word.size();
        for(int i = 0; i < len; ++i){
            if(p -> nex[word[i] - 'a'] == NULL){
                p -> nex[word[i] - 'a'] = new TrieNode();
            }
            p = p -> nex[word[i] - 'a'];
        }
        p -> isword = true;
    }
    
    /** Returns if the word is in the trie. */
    bool search(string word) {
        TrieNode *p = root;
        int len = word.size();
        for(int i = 0; i < len; ++i){
            if(p -> nex[word[i] - 'a'] == NULL) return false;
            p = p -> nex[word[i] - 'a'];
        }
        return p -> isword;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    bool startsWith(string prefix) {
        TrieNode *p = root;
        int len = prefix.size();
        for(int i = 0; i < len; ++i){
            if(p -> nex[prefix[i] - 'a'] == NULL) return false;
            p = p -> nex[prefix[i] - 'a'];
        }
        return true;
    }  
};

/**
 * Your Trie object will be instantiated and called as such:
 * Trie* obj = new Trie();
 * obj->insert(word);
 * bool param_2 = obj->search(word);
 * bool param_3 = obj->startsWith(prefix);
 */