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