为啥RE了？

#include <bits/stdc++.h>
using namespace std;
#define endl '\n'
int n,ans,m,sx,sy,tx,ty,flag;
typedef pair<int,int> PII;
const int N = 1100;
char g[N][N];
int vis[N][N];
int dx[] = {1,-1,0,0};
int dy[] = {0,0,1,-1};

int bfs(int x,int y){
    queue<PII> q;
    q.push({x,y});
    vis[x][y] = 1;
    while(q.size()){
        auto cur = q.front();
        q.pop();

        int x=cur.first,y=cur.second;
        for(int i=0;i<4;i++){
            int nx=x+dx[i],ny=y+dy[i];
            if(nx>=0&&nx<n&&ny>=0&&ny<n&&!vis[nx][ny]&&g[nx][ny] != g[x][y]){
                ans++;
                q.push({nx,ny});
                vis[nx][ny] = 1;
            }
        }
    }

}

int main()
{
    ios::sync_with_stdio(false),cin.tie(0),cout.tie(0);
    cin>>n>>m;
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
            cin>>g[i][j];
        }
    }
    while(m--){
        int i,j;cin>>i>>j;
        i--,j--;

        ans = 1;
        memset(vis,0,sizeof vis);
        bfs(i,j);
        cout << ans << endl;
    }
    return 0;
}