class Solution {
public:
    int n;
    vector<string> res;
    vector<string> generateParenthesis(int n) {
        this->n = n;
        dfs("", 0, 0);
        return res;
    }

    void dfs(string str, int left, int right) {
        if (left > n || right > left) return;
        else if (left == n && right == n) {
            res.push_back(str);
        } else {
            dfs(str + "(", left + 1, right);
            dfs(str + ")", left, right + 1);
        }
    }
};

换一种，这次数量是括号剩余个数，可以不传参 int n

class Solution {
public:
    vector<string> generateParenthesis(int n) {
        dfs("", n, n);
        return res;
    }
private:
    vector<string> res;
    void dfs(const string& str, int left, int right) {
        if (left < 0 || left > right)  // 出现类似 ()) )) 这种格式都是错误的不用再继续了
            return;
        if (left == 0 && right == 0) {
            res.push_back(str);
            return;
        }
        dfs(str + '(', left - 1, right);
        dfs(str + ')', left, right - 1);
    }
};