[CSES] Round Trip

void solve() {
    ll n, m;
    cin >> n >> m;

    vvll v(n);
    rep(i, m) {
        ll x, y;
        cin >> x >> y;
        --x, --y;
        v[x].pb(y), v[y].pb(x);
    }

    ll st = -1, et = -1;
    vll col(n, 0), p(n, -1);
    function <bool(ll, ll)> dfs = [&](ll node, ll pr) {
        debug(node);
        col[node] = 1, p[node] = pr;
        for (auto& child : v[node]) {
            if (child == pr) continue;
            if (col[child] == 0) {
                if (dfs(child, node) == 1) return 1;
            }
            else if (col[child] == 1) {
                st = child, et = node;
                return 1;
            }
        }
        col[node] = 2;
        return 0;
        };

    rep(i, n) {
        if (col[i] == 0) {
            if (dfs(i, i)) {
                debug(i);

                vll path;
                for (ll i = et; i != st; i = p[i]) path.pb(i);
                path.pb(st);
                path.pb(et);

                cout << sz(path) << "\n";
                rep(i, sz(path)) cout << path[i] + 1 << " ";
                cout << "\n";

                return;
            }
        }
    }

    cout << "IMPOSSIBLE\n";
}