Generate Parentheses

class Solution {
vector<string> res;
public:
void set(int left,int right,vector<string>& result,string str)
{
if(!left &&!right)
result.push_back(str);
if(left>0)
set(left-1,right,result,str+'(');
if(left<right &&right>0)
set(left,right-1,result,str+')');
}
vector<string> generateParenthesis(int n) {
string s;
set(n,n,res,s);
return res;
}
};