我有点看不懂题解的一个地方 就是为什么dfs时要搜边界 以下是一个题解

#include <cstdio>
using namespace std;
int n;
int a[32][32];
void dfs(int x, int y){
	if(x >= 0 && x <= n + 1 && y >= 0 && y <= n + 1){
		if(a[x][y] == 1 || a[x][y] == 3) return ;
		else{a[x][y] = 3;
			dfs(x + 1, y); dfs(x - 1, y);
			dfs(x, y + 1); dfs(x, y - 1);}
	}
}
int main(){
	scanf("%d", &n);
	for(int i = 1; i <= n; ++ i)
	for(int j = 1; j <= n; ++ j)
	scanf("%d", &a[i][j]);
	dfs(0, 0);
	for(int i = 1; i <= n; ++ i)
	for(int j = 1; j <= n; ++ j)
	if(a[i][j] == 3) a[i][j] = 0;
	else if(a[i][j] == 0) a[i][j] = 2;
	for(int i = 1; i <= n; ++ i, printf("\n"))
	for(int j = 1; j <= n; printf("%d ", a[i][j]), ++ j) ;
	return 0;
}