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