MLE求助，怎么样才不会MLE

#include<bits/stdc++.h>
using namespace std;
int n,a1,b1,a2,b2;
bool mapp[1005][1005];
queue<int>qx;
queue<int>qy;
queue<int>qz;
int bfs(){
	qx.push(a1);
	qy.push(b1);
	qz.push(0);
	int i=0;
	while(qx.empty()==0){
		int x=qx.front();
		int y=qy.front();
		int z=qz.front();
		qx.pop();
		qy.pop();
		qz.pop();
		if(x==a2&&y==b2){
			return z;
		}
		if(mapp[x+1][y]){
			qx.push(x+1);
			qy.push(y);
			qz.push(z+1);
		}
		if(mapp[x][y+1]){
			qx.push(x);
			qy.push(y+1);
			qz.push(z+1);
		}
		if(mapp[x-1][y]){
			qx.push(x-1);
			qy.push(y);
			qz.push(z+1);
		}
		if(mapp[x][y-1]){
			qx.push(x);
			qy.push(y-1);
			qz.push(z+1);
		}
	}
}
int main(){
	cin>>n;
	for(int i=1;i<=n;i++){
		for(int j=1;j<=n;j++){
			char c;
			cin>>c;
			mapp[i][j]=c-'0';
			mapp[i][j]=!mapp[i][j];
		}
	}
	cin>>a1>>b1>>a2>>b2;
	cout<<bfs();
	return 0;
}
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