小括号能够产生正确的对数

class Solution {
public:
    int num;
    vector<string> generateParenthesis(int n) {
        vector<string> res;
        if(n<=0){
            return res;
        }
        num=n;
        generate(0,0,"",res);
        return res;
        
    }
    // num 代表有多少对
    void generate(int left,int right,string str,vector<string> &res){
        if(left==num&&right==num){
            res.push_back(str);
            return ;
        }
        if(left<num){
            generate(left+1,right,str+'(',res);
        }
        if(right<num&&left>right){
            generate(left,right+1,str+')',res);
        }
        
    }
};