next.x = end_x;

     next.y = end_y;

     vis[end_x][end_y] = 2;      // 设置逆向探索标记为2

     q2[rear2++] = next;          // 终点坐标入逆向队列

     while (front1 < rear1 && front2<rear2)

     {

            if (rear1-front1 < rear2-front2)

                    {

                             cur = q1[front1++]; flag=1;    // 扩展正向队列

                    }

                    else

                    {

                             cur = q2[front2++]; flag=2;    // 扩展逆向队列

                    }

           for (i=0; i<8; ++i)

          {

             next.x = cur.x + dx[i];

             next.y = cur.y + dy[i];

             if (next.x<0 || next.x>=n || next.y<0 || next.y>=n)

                 continue;

             if (!vis[next.x][next.y])

             {

                 vis[next.x][next.y] = flag;    

                 step[next.x][next.y] = step[cur.x][cur.y] + 1;

                 if (flag==1)

                    q1[rear1++] = next;

                                      else