超时50分

求助怎样优化！！！？？？？

#include <bits/stdc++.h>
using namespace std;

struct Node{
	int x,y;
};

int n,m;
int mp[1005][1005];
bool vis[1005][1005];
char c[1005][1005];
int xx[]={-1,1,0,0};
int yy[]={0,0,-1,1};

int main(){
	cin>>n>>m;
	for(int i=1;i<=n;i++)
		for(int j=1;j<=n;j++)
			cin>>c[i][j];
	for(int i=1;i<=n;i++){
		for(int j=1;j<=n;j++){
			memset(vis,0,sizeof(vis));
			queue<Node> q;
			int ans=1;
			Node f,r;
			r={i,j};
			q.push(r);
			vis[i][j]=1;
			while(!q.empty()){
				f=q.front();
				q.pop();
				for(int i=0;i<4;i++){
					int dx=f.x+xx[i];
					int dy=f.y+yy[i];
					if(dx>=1&&dx<=n&&dy>=1&&dy<=n&&!vis[dx][dy]&&c[dx][dy]!=c[f.x][f.y]){
						vis[dx][dy]=1;
						r={dx,dy};
						q.push(r);
						ans++;
					}
				}
			}
			mp[i][j]=ans;
		}
	}
	for(int i=1;i<=m;i++){
		int t1,t2;
		cin>>t1>>t2;
		cout<<mp[t1][t2]<<endl;
	}
	return 0;
}