(11) : error C3861: “stoi”: 找不到标识符
(12) : error C2039: “back”: 不是“std::basic_string<_Elem,_Traits,_Ax>”的成员
        with
        [
            _Elem=char,
            _Traits=std::char_traits<char>,
            _Ax=std::allocator<char>
        ]