Node* merge(Node* l, Node* r) {
    if (l == nullptr || r == nullptr) return l ? l : r;
    if (l->h <= r->h) {
        r->l = merge(l, r->l);
        if (r->l != nullptr) r->l->p = r;
        r = balance(r);
        r->h = max(height(r->l), height(r->r)) + 1;
        r->size = (size(r->l) + size(r->r)) + 1;
        return r;
    } else {
        l->r = merge(l->r, r);
        if (l->r != nullptr) l->r->p = l;
        l = balance(l);
        l->h = max(height(l->l), height(l->r)) + 1;
        l->size = (size(l->l) + size(l->r)) + 1;
        return l;
    }
}