public void generateParenthesisDFS(int left, int right, String s) {
        if(right < left) return;
        if(left == 0 && right == 0){
            res.add(s);
        }
        if(left > 0){
            generateParenthesisDFS(left - 1, right, s + "(");
        }
        if(right > 0){
            generateParenthesisDFS(left, right - 1, s + ")");
        }
    }