LeetCode Add and Search Word - Data structure design

class CharNode {
public:
    int count;
    CharNode* child[26];
    CharNode(int cnt = 0) : count(cnt) {
        for (int i = 0; i<26; i++) {
            child[i] = NULL;
        }
    }
};

class WordDictionary {
private:
    CharNode root;
    void add(const string& str) {
        CharNode* current = &root;
        int pos = 0;
        int len = str.size();
        char ch;
        while (pos < len) {
            ch = str[pos] - ‘a‘;
            if (current->child[ch] == NULL) {
                current->child[ch] = new CharNode();
            }
            if (pos == len - 1) {
                current->child[ch]->count++;
            }
            current = current->child[ch];
            pos++;
        }
    }
    
    bool dfs_search(CharNode* at, string& word, int pos) {
        if (at == NULL) {
            return false;
        }
        int len = word.size();
        if (len == pos && at != NULL && at->count > 0) {
            return true;
        } else if (pos >= len) {
            return false;
        }
        if (word[pos] == ‘.‘) {
            // wildcard
            for (int i=0; i<26; i++) {
                if (dfs_search(at->child[i], word, pos + 1)) {
                    return true;
                }
            }
        } else {
            return dfs_search(at->child[word[pos] - ‘a‘], word, pos + 1);
        }
        return false;
    }
public:
    WordDictionary() {
        // placeholder for ‘\0‘ string
        root.count = 1;
    }
    // Adds a word into the data structure.
    void addWord(string word) {
        add(word);
    }

    // Returns if the word is in the data structure. A word could
    // contain the dot character ‘.‘ to represent any one letter.
    bool search(string word) {
        return dfs_search(&root, word, 0);
    }
};

// Your WordDictionary object will be instantiated and called as such:
// WordDictionary wordDictionary;
// wordDictionary.addWord("word");
// wordDictionary.search("pattern");