static SinglyLinkedListNode mergeLists(SinglyLinkedListNode head1, SinglyLinkedListNode head2) {
    SinglyLinkedListNode mergedHead = new SinglyLinkedListNode(0);
    SinglyLinkedListNode current = mergedHead;

    while (head1 != null && head2 != null) {
        if (head1.data <= head2.data) {
            current.next = head1;
            head1 = head1.next;
        } else {
            current.next = head2;
            head2 = head2.next;
        }
        current = current.next;
    }

    current.next = (head1 != null) ? head1 : head2;

    return mergedHead.next; 
}