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Leaving Cert Computer Science: Algorithmic complexity

How often Algorithmic complexity comes up on the Computer Science papers, every year it was asked, and questions to try.

HL Asked on 4 of the last 5 Higher Level papers, most recently in 2025. most years

Algorithmic complexity, Higher Level(7 marks)

Quick ones on Algorithmic complexity.

(a)What is the worst-case time complexity of linear search?
  1. O(log n)
  2. O(n)
  3. O(1)
(b)What is the worst-case time complexity of binary search?
  1. O(n²)
  2. O(n)
  3. O(log n)
(c)What is the worst-case time complexity of quicksort?
  1. O(n²)
  2. O(n)
  3. O(n log n)
Show the answers

(a) O(n)

(b) O(log n)

(c) O(n²)

Your turn: Higher Level questions on Algorithmic complexity.

Higher Level

Asked on 4 of the last 5 Higher Level papers, most recently in 2025. most years

Every paper, year by year

YearWhere it came up
2025B14
2024B14
2023B15
2022B14
2021Not asked

Links open the State Examinations Commission’s paper for that year.

More Algorithmic complexity questions

Algorithmic complexity, 3 marks

An O(n) algorithm takes 3 seconds for 10,000 items. About how long for 30,000 items?

  1. 27 seconds
  2. 3 seconds
  3. 9 seconds
Show the answer

9 seconds

O(n) time grows in proportion to n. Three times the items takes about three times as long: 3 x 3 = 9 seconds. 27 seconds is what O(n²) would give.

Algorithmic complexity, 2 marks

What does Big-O notation describe?

  1. How much memory the computer has for the input
  2. How an algorithm's steps grow as the input grows
  3. The exact running time of an algorithm in seconds
Show the answer

How an algorithm's steps grow as the input grows

Big-O ignores the speed of the machine. It shows the growth rate: e.g. an O(n) algorithm does about twice the work when the input doubles.

Other Computer Science topics

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