极简16行

#include<bits/stdc++.h>
using namespace std;
int n,m,u,v;
set<int>a[100005];
int main(){
    cin >> n >> m;
    while(m--){
        cin >> u >> v;
        a[u].insert(v);
        a[v].insert(u);
    }for(int i=1;i<=n;i++){
        cout << a[i].size()<<" ";
        for(auto it : a[i])cout << it << " ";
        cout << endl;
    }
}