def mergeLists(head1, head2):
    l, result = [], SinglyLinkedList()
    while head1:
        l.append(head1.data)
        head1 = head1.next
    while head2:
        l.append(head2.data)
        head2 = head2.next
    for data in sorted(l):
        result.insert_node(data)
    return result.head

def mergeLists(head1, head2):
    result = SinglyLinkedList()
    while head1 or head2:
        if not head1:
            result.insert_node(head2.data)
            head2 = head2.next
        elif not head2:
            result.insert_node(head1.data)
            head1 = head1.next
        elif head1 and head2:
            if head1.data <= head2.data:
                result.insert_node(head1.data)
                head1 = head1.next
            else:
                result.insert_node(head2.data)
                head2 = head2.next
    return result.head