Home iOS & Swift Books Data Structures & Algorithms in Swift

33
Heapsort Challenges Written by Vincent Ngo

Heads up... You're reading this book for free, with parts of this chapter shown beyond this point as scrambled text.

You can unlock the rest of this book, and our entire catalogue of books and videos, with a raywenderlich.com Professional subscription.

Challenge 1: Add heap sort to Array

Add a heapSort() method to Array. This method should sort the array in ascending order. A template has been provided for you in the starter playground.

Challenge 2: Theory

When performing a 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 really 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())

Have a technical question? Want to report a bug? You can ask questions and report bugs to the book authors in our official book forum here.

Have feedback to share about the online reading experience? If you have feedback about the UI, UX, highlighting, or other features of our online readers, you can send them to the design team with the form below:

© 2021 Razeware LLC

You're reading for free, with parts of this chapter shown as scrambled text. Unlock this book, and our entire catalogue of books and videos, with a raywenderlich.com Professional subscription.

Unlock Now

To highlight or take notes, you’ll need to own this book in a subscription or purchased by itself.