leetcode：括号生成

class Solution {
    vector<string> res;
    string str;
    void backtracking(int left,int right){
        if(left<0 || left>right) return;//出现无效情况就不继续
        if(left==0 && right==0){
            res.emplace_back(str);
            return;
        }
        if(left>0){//只在有效情况下继续
            str+='(';
            backtracking(left-1,right);
            str.pop_back();
        }
        if(right>left){
            str+=')';
            backtracking(left,right-1);
            str.pop_back();
        }
    }
public:
    vector<string> generateParenthesis(int n) {
        res.clear();
        str.clear();
        backtracking(n,n);
        return res;
    }
};