ADT from ADT (Cambridge International A-Level Computer Science)

Exam board Cambridge International · A-LevelUpdated 2025-05-19

ADT from ADT

What does it mean?

You should be able to explain or demonstrate how you can build one abstract data type on top of another
For example:
- A stack can be implemented using a linked list
- A queue can be implemented using two stacks
- A dictionary can be implemented using a binary search tree
You’re not just limited to built-in types like arrays, you can compose ADTs out of each other

What is an ADT?

If you are unsure what an ADT is, read our ADT overview page

How are ADTs implemented?

Using built-in types (e.g. arrays, lists)

ADT	Built-in type used	Example
Stack	List/array	Use `.append()` and `.pop()` in Python
Queue	List/array or circular buffer	Use `.append()` and `.pop(0)`
Linked list	Class with `next` fields	Implement nodes manually
Dictionary	Hash table (built-in `dict`)	Key-value mapping
Binary tree	Class with `left` and `right`	Each node links to children

Using other ADTs

ADT being built	Uses this ADT	Example
Queue	Two stacks	Enqueue in one stack, dequeue using a second stack
Stack	Linked list	Push/pop from head of list
Binary tree	Linked list or array	Nodes link to each other like a graph
Dictionary	Binary search tree	Store keys and values in BST for ordered lookup

Example: Implementing a Queue using Two Stacks

Idea:

Use two stacks (let’s say inStack and outStack) to simulate a queue

How it works:

enqueue(x) → push x onto inStack
dequeue() → if outStack is empty, pop all items from inStack and push onto outStack, then pop from outStack
This simulates FIFO behaviour using LIFO operations from stacks

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Exam Questions Flashcards

ADT

Built-in type used

Example

Stack

List/array

Use .append() and .pop() in Python

Queue

List/array or circular buffer

Use .append() and .pop(0)

Linked list

Class with next fields

Implement nodes manually

Dictionary

Hash table (built-in dict)

Key-value mapping

Binary tree

Class with left and right

Each node links to children

ADT being built

Uses this ADT

Example

Queue

Two stacks

Enqueue in one stack, dequeue using a second stack

Stack

Linked list

Push/pop from head of list

Binary tree

Linked list or array

Nodes link to each other like a graph

Dictionary

Binary search tree

Store keys and values in BST for ordered lookup