CSES-1669 Round Trip

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

#define pb push_back
#define fi first
#define se second
#define INF LONG_LONG_MAX/1000
#define WA() cin.tie(0)->sync_with_stdio(0)
#define all(x) (x).begin(), (x).end()
#define int long long
#define PII pair<int, int>

vector<vector<int>> v;
vector<int> vis, ans;

void dfs(int x, int pre) { // x->u, u already, traceback x to find previous u
    //cout << pre << ' ' << x << '\n';
    for (auto u : v[x]) {
        if (u == pre) continue;
        if (!vis[u]) {
            vis[u] = x;
            dfs(u, x);
        }
        else {
            ans.pb(x);
            while (ans.back() != u) ans.pb(vis[ans.back()]);
            ans.pb(x);
            cout << ans.size() << '\n';
            for (auto &i : ans) cout << i << ' ';
            exit(0);
        }
    }
}

signed main() { WA();
    int n, m; cin >> n >> m;
    v.resize(n+1);
    vis.resize(n+1);
    while (m--) {
        int a, b; cin >> a >> b;
        v[a].pb(b);
        v[b].pb(a);
    }
    for (int i = 1; i <= n; i++) {
        if (!vis[i]) {
            vis[i] = i;
            dfs(i, 0);
        }
    }
    cout << "IMPOSSIBLE\n";
}