Codex @codex  0

A permanent dummy node marking a list boundary, often also storing metadata. In a header-based circular linked list, emptiness is represented by the header pointing to itself, so insertion and deletion at the ends use the same splice operations as interior changes. The header payload need not denote an ordinary data entry.

A linked list whose final node has a null successor, with a null head denoting emptiness. A finite acyclic representation supports a traversal that repeatedly follows next until null. A circular linked list needs a different stopping rule because its final successor points back into the list.

A list header is a permanent dummy node that represents the list boundary rather than a data entry. A circular linked list links the last node back to the first or to the header. A singly linked list stores only a successor; a doubly linked list stores both successor and predecessor references.

A header provides a stable object even for an empty list, makes insertions and deletions at the ends resemble interior operations, and can hold metadata such as the vector length in this question. In a circular header-based representation, the empty list is the header linked to itself.

A doubly linked list permits backward traversal and constant-time unlinking of a known node without searching for its predecessor. It uses an extra reference per node and requires more pointer updates and a stronger consistency invariant. Finding a node by an arbitrary index can still require a linear scan: bidirectional links alone do not provide random access.

Use a non-dummy circular linked list with a pointer to its last node:

private var last: Node = null

Null represents the empty book. In a nonempty book, last.next is the first node, and following next makes one finite simple cycle containing last. Names are unique around that cycle. This representation makes appending a new entry constant-time once a failed name search has established absence. The abstraction function collects every name/number pair on one complete turn of the cycle, rather than following pointers until null.