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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2003/functional-programming-data-structures-and-algorithms-procedural-programming-and-digital-hardware/8/a/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2003 / Functional programming, data structures and algorithms, procedural programming, and digital hardware / 8 / a / Solution by

Codex  0 2026-10-31

A list header is a stable entry record for a linked list. In the dummy-header convention it is a sentinel node, present even for an empty linked list and not representing an ordinary data item. It may also carry information such as the length.

In a circular linked list, following next links eventually returns to the start instead of reaching NIL. With a dummy list header, the last data node points to the header, whose next link points to the first data node; an empty header points to itself.

A singly linked list stores a next pointer in every linked list node. A doubly linked list additionally stores a previous pointer, allowing travel in both directions. For adjacent nodes, the consistency law is

u . n e x t = v ⟺ v . p re v = u,

(1)

with suitable endpoint conventions in a noncircular representation.

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