2.Adjacency list

 #include <bits/stdc++.h>

#include <ext/pb_ds/assoc_container.hpp>

#include <ext/pb_ds/tree_policy.hpp>

using namespace std;

using namespace __gnu_pbds;

template <class type1>

#define ll long long int

#define endl "\n"

#define ordered_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>

using ordered_multiset = tree <type1, null_type, less_equal <type1>, rb_tree_tag, tree_order_statistics_node_update>;

 //ordered_multiset <ll> kek;(declaration for multiorder set)

// ordered_set o_set;(declaration)

//kek.order_of_key(i);(strictly less then i for multi order set)

//o_set.order_of_key(5) ;(strictly less then i for multi order set)

#define yes cout << "YES\n"

#define no cout << "NO\n"

#define mod 1000000007

ll POW(ll a,ll b)

{   ll ans=1;

   while(b>0)

   {   if(b%2)ans=(ans*a)%mod;

       b/=2;

       a=(a*a)%mod;

   }

   return ans;

}

const ll N=10000;

vector<ll>adj[N];

int main()

{

    ios_base::sync_with_stdio(0);

    cin.tie(0);

    cout.tie(0);

   

    ll n,m;

    cin>>n>>m;

    for(ll i=0;i<m;i++)

    {

         ll u,v;

         cin>>u>>v;

         adj[u].push_back(v);

         adj[v].push_back(u);

    }

        for(ll i=1;i<=n;i++)

    {

        for(ll j=0;j<adj[i].size();j++)cout<<adj[i][j]<<" ";

        cout<<endl;

    }

    return 0;

}