为二维数组动态分配内存。当程序编译时不能确定数组大小的时候，就需要动态分配数组空间， 此程序可以创建动态二维数组-For two-dimensional array of dynamic memory allocation. Compile-time when the procedure should not determine when the array size, we need dynamic allocation of array space, this procedure can create a dynamic two-dimensional array