扫四条边，不是四个角
```
/* 看了看题解，先DFS外面，外面的搞成一个数，再搞里面的就好了……NB */
#include <bits/stdc++.h>
using namespace std;
typedef long long lint;
lint n, a[31][31], i, j;
void dfs(lint x, lint y, lint toNum) {
if (x < 1 || x > n || y < 1 || y > n || a[x][y] != 0)
return;
a[x][y] = toNum;
dfs (x - 1, y, toNum);
dfs (x, y - 1, toNum);
dfs (x, y + 1, toNum);
dfs (x + 1, y, toNum);
}
int main() {
scanf ("%lld", &n);
for (i = 1; i <= n; i++)
for (j = 1; j <= n; j++)
scanf ("%lld", &a[i][j]);
for(int i=1;i<=n;i++){
dfs(1,i,3);
dfs(i,1,3);
dfs(n,i,3);
dfs(i,n,3);
}
for (i = 1; i <= n; i++) {
for (j = 1; j <= n; j++) {
if (a[i][j] == 0)
a[i][j] = 2;
if (a[i][j] == 3)
a[i][j] = 0;
printf ("%lld ", a[i][j]);
}
printf ("\n");
}
return 0;
}
by Null_h @ 2023-09-02 19:07:57
@[kdqs8218](/user/705712) 感谢！
by HappyDavid @ 2023-09-02 19:20:32
@[HappyDavid](/user/738761) AC了！
by HappyDavid @ 2023-09-02 19:28:11