求条（为啥别人都是WA就我是TLE）

#include<bits/stdc++.h>
using namespace std;
const int N=2003;
int T,n,m,a[N][N],ans=1000000009;
char ch;
bool vis[N][N];
string anstr;
struct node{
	int ai,aj,step;
	string way;
};
void bfs(){
	queue<node>q;
	q.push((node){1,1,0,""});
	while(q.size()){
		node arr=q.front();
		q.pop();
		int x=arr.ai,y=arr.aj,as=arr.step;
		if(vis[x][y])continue;
		string str=arr.way;
		if(x==n&&y==m){
			ans=min(ans,as);
			anstr=str;
		}
		
		vis[x][y]++;
		if(x-1>=1&&a[x][y]!=a[x-1][y])q.push((node){x-1,y,as+1,str+"U"});
		if(x+1<=n&&a[x][y]!=a[x+1][y])q.push((node){x+1,y,as+1,str+"D"});
		if(y-1>=1&&a[x][y]!=a[x][y-1])q.push((node){x,y-1,as+1,str+"L"});
		if(y+1<=m&&a[x][y]!=a[x][y+1])q.push((node){x,y+1,as+1,str+"R"});
		//BFS奥义
	}
}
int main(){
	ios::sync_with_stdio(false);
	cin>>T;
	while(T--){
		cin>>n>>m;
		for(int i=1;i<=n;i++){
			for(int j=1;j<=m;j++){
				cin>>ch;
				if(ch=='0')a[i][j]=0;
				else a[i][j]=1;
			}
		}
		bfs();
		if(ans==1000000009)cout<<-1<<endl;
		else cout<<ans<<endl<<anstr<<endl;
		ans=1000000009,anstr="";
		for(int i=1;i<=n;i++){
			for(int j=1;j<=m;j++){
				vis[i][j]=0;
				a[i][j]=0;
			}
		}
	}
	return 0;
}