static SinglyLinkedListNode mergeLists(SinglyLinkedListNode head1, SinglyLinkedListNode head2) {
        
        if (head1 is null) return head2;
        if (head2 is null) return head1;
        
        if (head1.data <= head2.data)
        {
            head1.next = mergeLists(head1.next, head2);
            return head1;
        }
        else
        {
          head2.next = mergeLists(head1, head2.next);
          return head2; 
        }
    }