def mergeLists(head1, head2): dummy = SinglyLinkedListNode(0) tail = dummy

while head1 and head2:
    if head1.data < head2.data:
        tail.next = head1
        head1 = head1.next
    else:
        tail.next = head2
        head2 = head2.next
    tail = tail.next

tail.next = head1 if head1 else head2
return dummy.next