算法——求n对（）有多少种输出方式？

    public List<String> generateParenthesis(int n) {
        List<String> res = new ArrayList<>();
        gen(res, n, n, "");
        return res;
    }

    private void gen(List<String> res, int left, int right, String str) {
        if (left == 0 && right == 0) {
            res.add(str);
            return;
        }
        if (left > 0) {
            gen(res, left - 1, right, str + "(");
        }
        if(right > left){
            gen(res, left, right - 1,str + ")");
        }
    }