我有一个疑问

#include<bits/stdc++.h>
#define int long long
using namespace std;
const int maxn = 15;
int n;
int a[maxn][maxn];
bool vis[maxn][maxn];
struct Node{
	int x, y, sum;
}d[1500];
int cnt, ans = -1;
void dfs(int tmp, int s)
{
	if(tmp > cnt)
	{
		ans = max(ans, s);
		return ;
	}
	int x = d[tmp].x;
	int y = d[tmp].y;
	if(vis[x - 1][y - 1] || vis[x - 1][y] || vis[x - 1][y + 1] || vis[x][y - 1] || vis[x][y] || vis[x][y + 1] || vis[x + 1][y - 1] || vis[x + 1][y] || vis[x + 1][y + 1])
		return ;
	dfs(tmp + 1, s);
	vis[x - 1][y - 1] = vis[x - 1][y] = vis[x - 1][y + 1] = vis[x][y - 1] = vis[x][y] = vis[x][y + 1] = vis[x + 1][y - 1] = vis[x + 1][y] = vis[x + 1][y + 1] = true;
	dfs(tmp + 1, s + d[tmp].sum);
	vis[x - 1][y - 1] = vis[x - 1][y] = vis[x - 1][y + 1] = vis[x][y - 1] = vis[x][y] = vis[x][y + 1] = vis[x + 1][y - 1] = vis[x + 1][y] = vis[x + 1][y + 1] = false;
}
signed main()
{
	cin >> n;
	for(int i = 1; i <= n; i++)
		for(int j = 1; j <= n; j++)
			cin >> a[i][j];
	for(int i = 2; i < n; i++)
	{
		for(int j = 2; j < n; j++)
		{
//			cout << i << ' ' << j << endl;
//			cout << a[i - 1][j - 1] <<' '<< a[i - 1][j] <<' ' << a[i - 1][j + 1] <<'\n';
//			cout <<a[i][j - 1] <<' '<< a[i][j] <<' '<< a[i][j + 1] <<'\n'; 
//			cout <<a[i + 1][j - 1] <<' '<< a[i + 1][j] <<' '<< a[i + 1][j + 1]<<'\n';
			d[++cnt].sum = a[i - 1][j - 1] + a[i - 1][j] + a[i - 1][j + 1] + 
			a[i][j - 1] + a[i][j] + a[i][j + 1] + 
			a[i + 1][j - 1] + a[i + 1][j] + a[i + 1][j + 1];
			d[cnt].x = i;
			d[cnt].y = j;
		}
	}
	dfs(1, 0);
	cout << ans;
	return 0;
}

#include<bits/stdc++.h>
#define int long long
using namespace std;
const int maxn = 15;
int n;
int a[maxn][maxn];
bool vis[maxn][maxn];
struct Node{
	int x, y, sum;
}d[1500];
int cnt, ans = -1;
void dfs(int tmp, int s)
{
	if(tmp > cnt)
	{
		ans = max(ans, s);
		return ;
	}
	int x = d[tmp].x;
	int y = d[tmp].y;
	dfs(tmp + 1, s);
	if(vis[x - 1][y - 1] || vis[x - 1][y] || vis[x - 1][y + 1] || vis[x][y - 1] || vis[x][y] || vis[x][y + 1] || vis[x + 1][y - 1] || vis[x + 1][y] || vis[x + 1][y + 1])
		return ;
	vis[x - 1][y - 1] = vis[x - 1][y] = vis[x - 1][y + 1] = vis[x][y - 1] = vis[x][y] = vis[x][y + 1] = vis[x + 1][y - 1] = vis[x + 1][y] = vis[x + 1][y + 1] = true;
	dfs(tmp + 1, s + d[tmp].sum);
	vis[x - 1][y - 1] = vis[x - 1][y] = vis[x - 1][y + 1] = vis[x][y - 1] = vis[x][y] = vis[x][y + 1] = vis[x + 1][y - 1] = vis[x + 1][y] = vis[x + 1][y + 1] = false;
}
signed main()
{
	cin >> n;
	for(int i = 1; i <= n; i++)
		for(int j = 1; j <= n; j++)
			cin >> a[i][j];
	for(int i = 2; i < n; i++)
	{
		for(int j = 2; j < n; j++)
		{
//			cout << i << ' ' << j << endl;
//			cout << a[i - 1][j - 1] <<' '<< a[i - 1][j] <<' ' << a[i - 1][j + 1] <<'\n';
//			cout <<a[i][j - 1] <<' '<< a[i][j] <<' '<< a[i][j + 1] <<'\n'; 
//			cout <<a[i + 1][j - 1] <<' '<< a[i + 1][j] <<' '<< a[i + 1][j + 1]<<'\n';
			d[++cnt].sum = a[i - 1][j - 1] + a[i - 1][j] + a[i - 1][j + 1] + 
			a[i][j - 1] + a[i][j] + a[i][j + 1] + 
			a[i + 1][j - 1] + a[i + 1][j] + a[i + 1][j + 1];
			d[cnt].x = i;
			d[cnt].y = j;
		}
	}
	dfs(1, 0);
	cout << ans;
	return 0;
}