nodo *alberobil(nodo *r, int k) {
    if (!r)
        return new nodo(k, 0);

    if (contaNodi(r->left) <= contaNodi(r->right)) {
        r->left = alberobil(r->left, k);
        return r;
    }

    r->right = alberobil(r->right, k);
    return r;
}