Generate Parentheses（组合，回溯）

class Solution {
private:
    char parenthesis[2];
    vector<string> res;
    int num;
public:
    void dfs(int dep,string temp){
        if(dep==num){
            stack<char> s;
            for (int i=0;i<temp.size();++i)
            {
                if(s.empty()) 
                    s.push(temp[i]);
                else if(!s.empty()&&temp[i]==‘)‘){
                    if(s.top()==‘(‘) 
                        s.pop();
                }else{
                    s.push(temp[i]);
                }
            }
            if(!s.empty()) return;
            res.push_back(temp);
            return;
        }
        for (int i=0;i<2;++i)
        {
            temp.push_back(parenthesis[i]);
            dfs(dep+1,temp);
            temp.pop_back();
        }
        return;
    }
    vector<string> generateParenthesis(int n) {
        parenthesis[0]=‘(‘;
        parenthesis[1]=‘)‘;
        num=n*2;
        string temp="";
        dfs(0,temp);
        return res;
    }
};