代码60分求助：

WA了四个点

#include<bits/stdc++.h>
using namespace std;
char a[1005][1005];
bool vis[1005][1005];
int n,m,sum=0,ans[1005],tong[1005][1005];
int dx[4]={0,0,1,-1};
int dy[4]={1,-1,0,0};
void dfs(int x,int y,int liannum){
	int nx,ny;
	for (int i=0;i<=3;i++){
		nx=x+dx[i];
		ny=y+dy[i];
		if (nx>=1&&nx<=n&&ny>=1&&ny<=n&&vis[nx][ny]==false&&a[nx][ny]!=a[x][y]){
			sum++;
			vis[nx][ny]=true;
			tong[nx][ny]=liannum;
			dfs(nx,ny,liannum);
		}
	}
	return ;
}

int main(){
	cin>>n>>m;
	for (int i=1;i<=n;i++){
		for (int j=1;j<=n;j++){
			cin>>a[i][j];
		}
	}
	int liannum=0;
	for (int i=1;i<=n;i++){
		for (int j=1;j<=n;j++){
			if (vis[i][j]==false){
				liannum++;
				sum=1;
				vis[i][j]=true;
				tong[i][j]=liannum;
				dfs(i,j,liannum);
				ans[liannum]=sum;
			}
		}
	}
	for (int i=1;i<=m;i++){
		int x,y;
		cin>>x>>y;
		cout<<ans[tong[x][y]]<<endl;
	}
	return 0;
}