public ArrayList<String> generateParenthesis(int n) {
        // Write your code here
        ArrayList<String> res = new ArrayList<String>();
        dfs(res, "", n, n);
        return res;
    }
    public void dfs(ArrayList<String> res, String s, int left, int right) {
        if (left > right) {
            return;
        }
        if (left == 0 && right == 0) {
            res.add(s);
            return;
        }
        if (left > 0) {
            dfs(res, s + '(', left - 1, right);
        }
        if (right > 0) {
            dfs(res, s + ')', left, right - 1);
        }
    }