Leetcode: Generate Parentheses

/*dfs回溯当left>right || left>0 || right>0 时可以减枝*/class Solution {public:    vector<string> generateParenthesis(int n) {        vector<string>answer;        dfs(answer,"",n,n);        return answer;    }    void dfs(vector<string> &answer,string ans,int left,int right){        if(left>right || left<0 || right<0){            return;        }        if(left==right && left==0){            answer.push_back(ans);        }        dfs(answer,ans+"(",left-1,right);        dfs(answer,ans+")",left,right-1);    }};