[CSES] Message 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);
        v[y].pb(x);
    }

    vll save(n, -1), vis(n, 0);
    function<void(ll) > bfs = [&](ll node) {
        queue<ll> q;
        q.push(node);

        while (sz(q)) {
            auto cur = q.front();
            q.pop();
            vis[cur] = 1;

            for (auto& child : v[cur]) {
                if (!vis[child]) {
                    q.push(child);
                    vis[child] = 1;
                    save[child] = cur;
                }
            }

        }
        };
    bfs(0);

    if (vis[n - 1]) {
        vll path;
        for (ll st = n - 1; st != 0; st = save[st])
            path.pb(st);
        path.pb(0);

        reverse(all(path));
        cout << sz(path) << "\n";
        rep(i, sz(path)) cout << path[i] + 1 << " ";
        cout << "\n";
    }
    else cout << "IMPOSSIBLE\n";
}