函数memset()用于给大量的存储空间设置初始值是方便快捷的。

数组around[]存储周围元素的相对下标，使得无序变有序，可以用循环来处理。

这个是程序是之前写的，直接拿过来用就AC了(The function memset () is convenient and quick for setting initial values to a large number of storage spaces.

The array around[] stores the relative index of the surrounding elements, so that the disorder becomes ordered and can be processed by loops.

This is the program is written before, take it directly to use AC)