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