括号生成-力扣

class Solution {
private:
    vector<string> result;
    string str;
public:
    void backtracking(int n, int l, int r){
        if(l == n && r == n){
            result.push_back(str);
            return;
        }
        if(l < n){
            str.push_back('(');
            backtracking(n, l + 1, r);
            str.pop_back();
        }
        if(r < l){
            str.push_back(')');
            backtracking(n, l, r + 1);
            str.pop_back();
        }
    }
    vector<string> generateParenthesis(int n) {
        int l = 0, r = 0;
        backtracking(n, l, r);
        return result;
    }
};