public void addWord(String word) {
        TrieNode ptr = root;
        for (char c : word.toCharArray()) {
            if (ptr.children[c - 'a'] == null) {
                ptr.children[c - 'a'] = new TrieNode();
            }
            ptr = ptr.children[c - 'a'];
        }
        ptr.end = true;
    }