leetcode generate parentheses

class  {
public:
    void dfs(vector<string> &res, string v, int n, int left, int right) {
        if(n == left) {
            v.append(n - right, ')');
            res.push_back(v);
            return;
        }
        dfs(res, v + '(', n, left + 1, right);
        if(left > right) dfs(res, v + ')', n, left, right + 1);
    }
    vector<string> generateParenthesis(int n) {
        vector<string> res;
        if(n > 0) dfs(res, "", n, 0, 0);
        return res;
    }
};