leetcode generate parenthese

class Solution {public:    void dfs(int leftnum, int rightnum,const int totpair, vector<string>&ret, string str){        if(leftnum == totpair){            for(int iter = rightnum; iter < totpair; ++iter){                str += ")";            }            ret.push_back(str);            return;        }        dfs(leftnum + 1, rightnum, totpair, ret, (string)(str + "("));        if(leftnum > rightnum){            dfs(leftnum, rightnum + 1, totpair, ret, (string)(str + ")"));        }        return;    }    vector<string> generateParenthesis(int n) {        vector<string> ret;        if(n == 0){            return ret;        }        if(n == 1){            ret.push_back("()");            return ret;        }        string str = "";        dfs( 0, 0, n,ret, str);        return ret;    }};