def mergeLists(head1, head2):
    
    merged = SinglyLinkedList()
    
    while head1 is not None and head2 is not None:
        if head1.data < head2.data:
            merged.insert_node(head1.data)
            head1 = head1.next
        else:
            merged.insert_node(head2.data)
            head2 = head2.next
            
    if head1 is not None:
        while head1 is not None:
            merged.insert_node(head1.data)
            head1 = head1.next
            
    if head2 is not None:
        while head2 is not None:
            merged.insert_node(head2.data)
            head2 = head2.next
            
    return merged.head