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