LeetCode Tries Prefix Tree

class TrieNode {
public:
    const static int NR_FANOUT = 26;
    TrieNode* child[NR_FANOUT];
    int count;
    // Initialize your data structure here.
    TrieNode() {
        for (int i=0; i<NR_FANOUT; i++) {
            child[i] = NULL;
        }
        count = 0;
    }
};

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

    // Inserts a word into the trie.
    void insert(string s) {
        insert(s, 0, root);
    }

    // Returns if the word is in the trie.
    bool search(string key) {
        return search(key, 0, root) != NULL;
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    bool startsWith(string prefix) {
        return search_prefix(prefix, 0, root) != NULL;
    }
private:
    TrieNode* insert(string& s, int pos, TrieNode* current) {
        int len = s.size();
        if (pos > len) {
            return NULL;
        }
        if (current == NULL) {
            current = new TrieNode();
        }
        if (pos == len) {
            current->count++;
            return current;
        }
        int idx = s[pos] - 'a';
        current->child[idx] = insert(s, pos + 1, current->child[idx]);
        return current;
    }

    TrieNode* search(string& s, int pos, TrieNode* current) {
        if (current == NULL) {
            return NULL;
        }
        int len = s.size();
        if (pos > len) {
            return NULL;
        } else if (pos == len && current->count) {
            return current;
        }
        return search(s, pos + 1, current->child[s[pos] - 'a']);
    }
    TrieNode* search_prefix(string& s, int pos, TrieNode* current) {
        if (current == NULL) {
            return NULL;
        }
        int len = s.size();
        if (pos >= len) {
            return current;
        }
        return search_prefix(s, pos + 1, current->child[s[pos] - 'a']);
    }
private:
    TrieNode* root;
};

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

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