力扣 1021 刪除最外層括號*********

class Solution {
public:
    string removeOuterParentheses(string S) {
        stack<char> sta;
        string res;
        for(auto c : S)
        {
            if(c==')')  sta.pop();            
            if(!sta.empty())  res.push_back(c);

            if(c=='(') sta.push(c);
        }
        return res;
    }
};