code: void refresh_stack_information()
      {
          int sz = stack.size();
          if (0 == sz)
          {
              top0 = -1;
              top1 = -1;
          }
          else if (1 == sz)
          {
              top0 = stack.get(sz - 1);
              top1 = -1;
          }
          else
          {
              top0 = stack.get(sz - 1);
              top1 = stack.get(sz - 2);
          }
      }
docs: 更新栈的信息