static SinglyLinkedListNode mergeLists(SinglyLinkedListNode head1, SinglyLinkedListNode head2) {
    SinglyLinkedListNode l = null;
    SinglyLinkedListNode n1 = head1;
    SinglyLinkedListNode n2 = head2;
    if(n1==null & n2==null) return null;
    if(n1==null) return n2;
    if(n2==null) return n1;
    if(n1.data<=n2.data){
        l = n1;
        l.next = mergeLists(n1.next,n2);
    }else{
        l = n2;
        l.next = mergeLists(n1,n2.next);
    }

return l;
}