22 括号生成

https://leetcode.cn/leetbook/read/top-interview-questions-medium/xv33m7/

cost: 6m3s

class Solution {
public:
    vector<string> generateParenthesis(int n) {
        vector<string> res;
        string s = "";
        this->walk(res, s, n, n);
        return res;
    }

private:
    void walk(vector<string>& res, string& s, int n, int n2) {
        if (n == 0 && n2 == 0) {
            res.push_back(s);
            return;
        }w
        if (n < n2) {
            s.push_back(')');
            this->walk(res, s, n, n2 - 1);
            s.pop_back();
        }
        if (n > 0) {
            s.push_back('(');
            this->walk(res, s, n - 1, n2);
            s.pop_back();
        }
    }
};