// Lists is sorted in descending order.
bool inserted = false;
for (int i=0; i<lists.size(); ++i) {
    if (lists[i]->val < newNode->val) {
        if (i == 0) {
            inserted = true;
            lists.insert(lists.begin(), newNode);
            break;
        } else {
            inserted = true;
            lists.insert(lists.begin() + i - 1, newNode);
            break;
        }
    }
}
if (!inserted) {
    lists.push_back(newNode);
}