RoundTrip.cpp

#include<bits/stdc++.h>
using namespace std;


#define ll long long 
#define ar array

const int mxN = 1e5;
int n, m, p[mxN];
vector<int> adj[mxN], ans;
bool vis[mxN];

void dfs(int u, int pu = -1){
    p[u] = pu;
    vis[u] = 1;
    for(int v : adj[u]){
        if(v==pu)
           continue;
        if(vis[v]){
            int u2 = u;
            while(u^v){
                ans.push_back(u);
                u = p[u];
            }
            ans.push_back(v);
            ans.push_back(u2);
            cout << ans.size() << "\n";
            for(int a: ans)
               cout << a+1 << " ";
            exit(0);

        }else
           dfs(v, u);
    }
}

int main(){
    cin >> n >>  m;
    for(int i=0, a,b; i<m; ++i){
        cin >> a >> b, --a, --b;
        adj[a].push_back(b);
        adj[b].push_back(a);
    }
    for(int i=0; i<n; ++i){
        if(!vis[i]){
            dfs(i);
        }
    }
    cout << "IMPOSSIBLE";
}