80分RE怎么办？？？

#include<bits/stdc++.h>
using namespace std;
vector<int>a[2005];
int n,m;
bool vis[2005];
int main(){
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>m;
		if(m>0){
			int c;
			for(int j=1;j<=m;j++){
				cin>>c;
				vis[c]=1;
				a[i].push_back(c);
			}
		}
		else{
			int c;
			cin>>c;
			a[i].push_back(c);
			if(c>0)vis[c]=1;
		}
	}
	int first;
	for(int i=1;i<=n;i++){
		if(vis[i]==0)first=i;
	}
	while(1){
		cout<<first<<" ";
		int len=a[first].size();
		for(int i=0;i<len;i++)cout<<a[first][i]<<" ";
		first=a[a[first][len-1]][0];
		if(first==(-1))return 0;
	}
	return 0;
}