IB Computer Science HL · Year 2 · Lesson 23

Doubly and circular linked lists

Abstract data types separate what operations mean from how they are implemented, helping you choose structures by behaviour and trade-offs.

Paper 250 minutesB4.1.3
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Today’s targets

What you need to be able to do

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  • Construct + applyAnd apply linked lists: singly, doubly and circular
  • RetrieveReconnect today’s new material to previously taught content from this topic.
B4.1.3
Paper 2 lensMatch the depth of every response to the command term. Previously learned content can move quickly, but retrieval must still be accurate.
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Retrieve

Rapid Recall Deck

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Say the answer aloud before flipping. Mark secure knowledge quickly and spend time on the gaps.

  • What extra link does a doubly linked-list node have?: A reference to the previous node in addition to the reference to the next node.
  • What is the main benefit of a doubly linked list?: It supports traversal in both directions and can simplify some insertions/deletions when a node is already known.
  • What is the memory trade-off of a doubly linked list?: Each node stores an additional reference, increasing memory use and link-management complexity.
  • What makes a linked list circular?: The final node links back to the first node instead of ending with a null/no-next reference.
  • When is a circular linked list useful?: When processing should cycle repeatedly through items, such as round-robin scheduling or repeating playlists.
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B4.1.3 · Learn

Core knowledge and application

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B4.1.3Construct + apply

And apply linked lists: singly, doubly and circular

Abstract data types separate what operations mean from how they are implemented, helping you choose structures by behaviour and trade-offs.

Exam moveBuild the structure/solution and use it correctly in a new context.
Required detail 1The basic operations on a linked list, such as insertion, deletion, traversal, search
Assessment boundaryPaper 2

Explain it without notes

Construct + apply: And apply linked lists: singly, doubly and circular in the context of a system that needs efficient insertion, search and set membership.

  • The basic operations on a linked list, such as insertion, deletion, traversal, search
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Worked example · B4.1.2 / B4.1.3

Doubly and circular linked lists

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Doubly linked

null←
Aprev/next
↔
Bprev/next
↔
Cprev/next
→null

Each node can reach both its predecessor and successor, making reverse traversal possible but requiring extra pointer storage and more updates during insertion/deletion.

Circular linked

head→
Anext
→
Bnext
→
Cnext=head
↺

The final node links back to the first node. Traversal therefore needs a deliberate stopping condition.

ApplyChoose singly, doubly or circular for: browser back/forward navigation, round-robin player turns, and a lightweight forward-only queue. Justify each choice.
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Apply

Transfer to a new scenario

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ScenarioA system that needs efficient insertion, search and set membership needs a design or technical decision related to today’s topic. Explain what matters and why.
  • The basic operations on a linked list, such as insertion, deletion, traversal, search
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Exam lens

Paper 2 practice

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Build the response before checking notesUse precise terminology and match the required depth.
  1. Construct + apply: And apply linked lists: singly, doubly and circular in the context of a system that needs efficient insertion, search and set membership.
Self-checkAnswer the exact command term. For explain, include mechanism/reason; for compare, pair criteria; for discuss/evaluate/justify, build supported reasoning and a conclusion.
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Homework

Finish the learning cycle

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IA — main task

Continue Criterion D development. Keep code readable, record meaningful implementation decisions, and maintain testing/evidence notes as you work.

Syllabus — short

Complete one targeted Paper 2/Paper 1 retrieval task from today’s lesson.

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End of Lesson 23.