22. 括号生成/C++

class Solution {
public:
    vector<string> res;
    vector<string> generateParenthesis(int n) {
        generate("",0,0,n);
        return res;
    }
    void generate(string str, int left, int right, int max){
        if(str.size()==2*max){
            res.push_back(str);
            return;
        }
        if(left<max) generate(str+"(",left+1,right,max);
        if(right<left) generate(str+")",left,right+1,max);
    }
};