经检验，问题出在bfs（）函数中。

#include<bits/stdc++.h>
using namespace std;
int n;
int dx[4]={0,0,-1,1};
int dy[4]={1,-1,0,0};
int vis[35][35];
char s[35][35];
struct node{
	int x;
	int y;
};
queue<node>q;
void bfs(int x,int y){
	q.push({x,y});
	while(!q.empty()){
		node a=q.front();
		q.pop();
		for(int i=0;i<4;++i){
			int nx=a.x+dx[i];
			int ny=a.y=dy[i];
			if(nx>=1&&nx<=n&&ny>=1&&ny<=n&&vis[nx][ny]==0&&s[nx][ny]=='0'){
				vis[nx][ny]=1;
				s[nx][ny]='2';
				q.push({nx,ny});
			}
		}
	}
}
int main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	cin>>n;
	for(int i=1;i<=n;++i)
		for(int j=1;j<=n;++j)
			cin>>s[i][j];
	for(int i=1;i<=n;++i)
		for(int j=1;j<=n;++j)
			if(i==1&&s[i][j]=='0'||i==n&&s[i][j]=='0'||j==1&&s[i][j]=='0'||j==n&&s[i][j]=='0') /*靠边的‘0’未被包裹*/
				vis[i][j]=-1;
	for(int i=1;i<=n;++i)
		for(int j=1;j<=n;++j)
			if(vis[i][j]!=-1){
				vis[i][j]=1;
				bfs(i,j);
				break;
			}
	for(int i=1;i<=n;++i,cout<<'\n')
		for(int j=1;j<=n;++j)
			if(vis[i][j]==-1){
				s[i][j]='0';
				cout<<s[i][j]<<' ';
			}else	cout<<s[i][j]<<' ';
	return 0;
}