Heapsort is a comparison-based

Comparison sort

A comparison sort is a type of sorting algorithm that only reads the list elements through a single abstract comparison operation that determines which of two elements should occur first in the final sorted list...

 sorting algorithm

Sorting algorithm

In computer science, a sorting algorithm is an algorithm that puts elements of a list in a certain order. The most-used orders are numerical order and lexicographical order...

 to create a sorted array

Sorted array

A sorted array is an array data structure in which each element is sorted in numerical, alphabetical, or some other order, and placed at equally spaced addresses in computer memory. It is typically used in computer science to implement static lookup tables to hold multiple values which has the same...

 (or list), and is part of the selection sort

Selection sort

Selection sort is a sorting algorithm, specifically an in-place comparison sort. It has O time complexity, making it inefficient on large lists, and generally performs worse than the similar insertion sort...

 family. Although somewhat slower in practice on most machines than a well implemented quicksort, it has the advantage of a more favorable worst-case O

Big O notation

In mathematics, big O notation is used to describe the limiting behavior of a function when the argument tends towards a particular value or infinity, usually in terms of simpler functions. It is a member of a larger family of notations that is called Landau notation, Bachmann-Landau notation, or...

(n log n) runtime. Heapsort is an in-place algorithm

In-place algorithm

In computer science, an in-place algorithm is an algorithm which transforms input using a data structure with a small, constant amount of extra storage space. The input is usually overwritten by the output as the algorithm executes...

, but is not a stable sort.
Heapsort begins by building a heap

Heap (data structure)

In computer science, a heap is a specialized tree-based data structure that satisfies the heap property: if B is a child node of A, then key ≥ key. This implies that an element with the greatest key is always in the root node, and so such a heap is sometimes called a max-heap...

 out of the data set, and then removing the largest item and placing it at the end of the partially sorted array.