[CSES] Longest Flight Route

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 topo, vis(n, 0);
    function <void(ll, ll)> dfs = [&](ll node, ll pr) {
        vis[node] = 1;
        for (auto& child : v[node]) {
            if (!vis[child]) dfs(child, node);
        }
        topo.pb(node);
        };
    dfs(0, 0);
    reverse(all(topo));

    vll dp(n, -inf), save(n, -1);
    dp[0] = 0;
    for (auto& node : topo) {
        for (auto& child : v[node]) {
            dp[child] = max(dp[child], dp[node] + 1);
            if (dp[child] == dp[node] + 1) save[child] = node;
        }
    }

    if (dp[n - 1] == -inf) {
        cout << "IMPOSSIBLE\n";
        return;
    }

    vll path;
    for (ll i = n - 1; ; i = save[i]) {
        path.pb(i);
        if (i == 0) break;
    }
    reverse(all(path));

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