[CSES] Round Trip II

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);
    }

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

                vll cycle;
                cycle.pb(st);
                while (st ^ et) {
                    cycle.pb(et);
                    et = p[et];
                }
                cycle.pb(st);
                reverse(all(cycle));

                cout << sz(cycle) << "\n";
                for (auto& x : cycle) cout << x + 1 << " ";
                cout << "\n";


                return 1;
            }
        }
        col[node] = 2;
        return 0;
        };

    rep(i, n) {
        if (col[i] == 0) {
            if (dfs(i, i)) {
                return;
            }
        }
    }
    cout << "IMPOSSIBLE\n";
}