class Solution {public:    bool isValid(const string &s)    {        stack<char> stk;        for (int i = 0; i < s.length(); i++)        {            switch (s[i])            {            case ‘(‘:                 stk.push(‘(‘);                break;            case ‘)‘ :                if (!stk.empty())                {                    if (stk.top() == ‘(‘)                    {                        stk.pop();                        continue;                    }                                    }                return false;                break;            }        }        return stk.empty();    }    void go(string s, int n, vector<string> &ret)    {        if (s.length() == n * 2 )        {            if (isValid(s))        ret.push_back(s);            return;        }        go(s + "(", n, ret);        go(s + ")", n, ret);    }    vector<string> generateParenthesis(int n) {        vector<string> rec;        go("(", n, rec);        return rec;    }};