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Beginner Topic 12 of 51 Algorithms

Quick Sort

Partitioning around a pivot then sorting each side.

How it works

Quick sort picks a pivot, partitions the array so that smaller elements come before it and larger ones after, then recursively sorts both sides. It averages O(n log n) with excellent cache behavior and small constants, sorts in place, but degrades to O(n squared) with consistently bad pivots, and the usual version is not stable. Median-of-three or random pivots avoid most bad cases.

Where it is used

General-purpose in-place sorting in C and C++ libraries, introsort implementations, selecting the k-th smallest element with quickselect and sorting large arrays in memory use quicksort.

Common pitfall

A poor pivot choice on sorted or equal-valued input causes quadratic time and deep recursion that can overflow the stack. Partition bugs with duplicates or index crossing are common. Recurse on the smaller side first, use three-way partitioning for many duplicates and fall back to heapsort in worst cases.

Where it fits in the course

Quick Sort is topic 12 of 51 in Algorithms, rated Beginner. It follows Merge Sort. The next topic is Counting Sort.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Algorithms quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Quick Sort?
Partitioning around a pivot then sorting each side. It belongs to Algorithms and is a beginner-level topic.
Where do people use Quick Sort?
General-purpose in-place sorting in C and C++ libraries, introsort implementations, selecting the k-th smallest element with quickselect and sorting large arrays in memory use quicksort.
What is the most common mistake with Quick Sort?
A poor pivot choice on sorted or equal-valued input causes quadratic time and deep recursion that can overflow the stack.
What should I learn before this topic?
Start with Merge Sort, the topic just before it in Algorithms.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Algorithms quiz. You can also explore Sorting Algorithms in the Visual Lab.
Is Quick Sort hard to learn?
It is a Beginner topic, so it works as an entry point. If the terms are new, read the explanation twice and try the visualizer before the quiz.
How long does it take to learn Quick Sort?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Quick Sort?
Merge Sort, Counting Sort, Insertion Sort are closely connected. The full list is in the related topics section of this page.
What should I study after Quick Sort?
Continue with Counting Sort, the next topic in Algorithms.

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