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32
Heapsort Written by Vincent Ngo

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Heapsort is another comparison-based algorithm that sorts an array in ascending order using a heap. This chapter builds on the heap concepts presented in Chapter 22, “The Heap Data Structure.”

Heapsort takes advantage of a heap being, by definition, a partially sorted binary tree with the following qualities:

  1. In a max heap, all parent nodes are larger than their children.
  2. In a min heap, all parent nodes are smaller than their children.

The diagram below shows a heap with parent node values underlined:

Getting started

Open up the starter playground. This playground already contains an implementation of a max heap. Your goal is to extend Heap so it can also sort. Before you get started, let’s look at a visual example of how heap sort works.

Example

For any given unsorted array, to sort from lowest to highest, heap sort must first convert this array into a max heap:

Implementation

Next, you’ll implement this sorting algorithm. The actual implementation is very simple, as the heavy lifting is already done by the siftDown method:

extension Heap {
  func sorted() -> [Element] {
    var heap = Heap(sort: sort, elements: elements) // 1
    for index in heap.elements.indices.reversed() { // 2
      heap.elements.swapAt(0, index) // 3
      heap.siftDown(from: 0, upTo: index) // 4
    }
    return heap.elements
  }
}
let heap = Heap(sort: >, elements: [6, 12, 2, 26, 8, 18, 21, 9, 5])
print(heap.sorted())
[2, 5, 6, 8, 9, 12, 18, 21, 26]

Performance

Even though you get the benefit of in-memory sorting, the performance of heap sort is O(n log n) for its best, worse and average cases. This is because you have to traverse the whole list once and, every time you swap elements, you must perform a sift down, which is an O(log n) operation.

Key points

  • Heap sort leverages the max-heap data structure to sort elements in an array.
  • Heap sort sorts its elements by following a simple pattern:

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