CSES – Round Trip

#include<bits/stdc++.h>
#define int long long
#define f first
#define s second
#define fastio ios_base::sync_with_stdio(0);cin.tie(0)
using namespace std;
//declare
const int maxn = 1e5+5;
int n,m;
bool vis[maxn];
vector<int> v, G[maxn];
//
void dfs(int cur,int par){
    if(vis[cur]){
        bool flag = 0;
        v.push_back(cur);
        vector<int> ans;
        for(int i:v){
            if(i == cur) flag = 1;
            if(flag) ans.push_back(i);
        }
        cout<<ans.size()<<"\n";
        for(int i:ans) cout<<i<<" ";
        cout<<"\n";
        exit(0);
    }
    vis[cur] = 1;
    v.push_back(cur);
    for(int adj : G[cur]){
        if(adj == par) continue;
        dfs(adj,cur);
    }
    v.pop_back();
}
signed main(){
    fastio;
    cin>>n>>m;
    for(int i=0;i<m;i++){
        int a,b; cin>>a>>b;
        G[a].push_back(b);
        G[b].push_back(a);
    }
    memset(vis,0,sizeof(vis));
    for(int i=1;i<=n;i++){
        if(!vis[i]){
            v.clear();
            dfs(i,-1);
        }
    }
    cout<<"IMPOSSIBLE\n";
    return 0;
}