Generate Parentheses

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