bool isOperator(string &op)

      {

          //注意用法

          return op.size() ==  && string("+-*/").find(op) != string::npos;

      }

      int evalRPN(vector<string> &tokens)

      {

          stack<string> s;

          for (auto token : tokens)

          {

              if (!isOperator(token))

              {

                  //如果是操作数，则入栈

                  s.push(token);

              }

              else

              {

                  //如果是操作符，则弹出操作数进行运算

                  int y = stoi(s.top());

                  s.pop();

                  int x = stoi(s.top());

                  s.pop();

                  if (token == "+")x += y;

                  if (token == "-")x -= y;

                  if (token == "*")x *= y;

                  if (token == "/")x /= y;

                  s.push(to_string(x));

              }

          }

          return stoi(s.top());

      }