## Data Structures & Algorithms in Swift

Fourth Edition · iOS 15 · Swift 5.5 · Xcode 13

#### Before You Begin

Section 0: 6 chapters

#### Section I: Introduction

Section 1: 3 chapters

#### Section II: Elementary Data Structures

Section 2: 6 chapters

# 33. Heapsort Challenges Written by Vincent Ngo

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### Challenge 1: Add heap sort to Array

Add a heapSort() method to Array. This method should sort the array in ascending order. A starting template is in the starter playground.

### Challenge 2: Theory

When performing heapsort in ascending order, which of these starting arrays requires the fewest comparisons?

### Challenge 3: Descending order

The current implementation of heapsort in Chapter 32 sorts the elements in ascending order. How would you sort in descending order?

## Solutions

### Solution to Challenge 1

To add heap sort to Array, you must create an extension, where the elements in the array must be Comparable. Everything else is straightforward as the implementation is similar to the Heap in Chapter 32.

extension Array where Element: Comparable {

func leftChildIndex(ofParentAt index: Int) -> Int {
(2 * index) + 1
}

func rightChildIndex(ofParentAt index: Int) -> Int {
(2 * index) + 2
}

mutating func siftDown(from index: Int, upTo size: Int) {
var parent = index
while true {
let left = leftChildIndex(ofParentAt: parent)
let right = rightChildIndex(ofParentAt: parent)
var candidate = parent

if (left < size) && (self[left] > self[candidate]) {
candidate = left
}
if (right < size) && (self[right] > self[candidate]) {
candidate = right
}
if candidate == parent {
return
}
swapAt(parent, candidate)
parent = candidate
}
}

mutating func heapSort() {
// Build Heap
if !isEmpty {
for i in stride(from: count / 2 - 1, through: 0, by: -1) {
siftDown(from: i, upTo: count)
}
}

// Perform Heap Sort.
for index in indices.reversed() {
swapAt(0, index)
siftDown(from: 0, upTo: index)
}
}
}

### Solution to Challenge 2

When sorting elements in ascending order using heap sort, you first need a max heap. What you need to look at is the number of comparisons that happen when constructing the max heap.

### Solution to Challenge 3

Simply use a min heap instead of a max heap before sorting:

let heap = Heap(sort: <, elements: [6, 12, 2, 26, 8, 18, 21, 9, 5])
print(heap.sorted())
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