def mergeLists(head1, head2): dummy=SinglyLinkedListNode(0) cur=dummy while head1 and head2:

    if head1.data<=head2.data:
        cur.next=head1
        head1=head1.next
    else:
        cur.next=head2
        head2=head2.next
    cur=cur.next
if head1:
    cur.next=head1
if head2:
    cur.next=head2
return dummy.next