function mergeLists(head1, head2) {
    const list = [];
    while (head1) {
        list.push(head1.data);
        head1 = head1.next;
    }
    while (head2) {
        list.push(head2.data);
        head2 = head2.next;
    }
    list.sort((a, b) => a - b);
    const head = new SinglyLinkedListNode(list[0]);
    list.reduce((node, value, index) => {
        if (index) {
            node.next = new SinglyLinkedListNode(value);
            node = node.next;
        }
        return node;
    }, head);
    return head;
}