Implement Trie (Prefix Tree)

Implement a trie with insert, search, and startsWith methods.

Note:
You may assume that all inputs are consist of lowercase letters a-z.

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Just code

const int N = 26;
class TrieNode {
public:
    // Initialize your data structure here.
    TrieNode* children[N];
    bool iskey;
    TrieNode() {
        for(size_t i=0; i<N; i++){
            children[i] = NULL;
        }
        iskey = false;
    }
};

class Trie {
public:
    Trie() {
        root = new TrieNode();
    }

    // Inserts a word into the trie.
    void insert(string word) {
        TrieNode *p = root;
        for(size_t i=0; i<word.size(); i++){
            int ci = word[i]-'a';
            if(!p){
                p = new TrieNode();
            }
            if(p->children[ci] == NULL){
                p->children[ci] = new TrieNode();
            }
            p = p->children[ci];
        }
        p->iskey = true;
    }

    bool search(string word) {
        TrieNode *p = root;
        for(size_t i=0; i<word.size(); i++){
            int ci = word[i]-'a';
            if(!p || p->children[ci] == NULL){
                return false;
            }
            p = p->children[ci];
        }
        return p && p->iskey;
    }

    bool startsWith(string prefix) {
        TrieNode *p = root;
        for(size_t i=0; i<prefix.size(); i++){
            int ci = prefix[i]-'a';
            if(!p || p->children[ci] == NULL){
                return false;
            }
            p = p->children[ci];
        }
        return true;
    }

private:
    TrieNode* root;
};

// Your Trie object will be instantiated and called as such:
// Trie trie;
// trie.insert("somestring");
// trie.search("key");