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Data Structures & Algorithms in Swift

Fifth Edition · iOS 18 · Swift 6.0 · Xcode 16.2

Before You Begin

Section 0: 6 chapters
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Section I: Introduction

Section 1: 3 chapters
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Section II: Elementary Data Structures

Section 2: 6 chapters
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Section III: Trees

Section 3: 16 chapters
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Section IV: Sorting Algorithms

Section 4: 10 chapters
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Section V: Graphs

Section 5: 11 chapters
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9. Queue Challenges
Written by Vincent Ngo

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Think you have a handle on queues? In this chapter, you will explore five different problems related to queues. This serves to solidify your fundamental knowledge of data structures in general.

Challenge 1: Stack vs. Queue

Explain the difference between a stack and a queue. Provide two real-life examples for each data structure.

Challenge 2: Step-by-step Diagrams

Given the following queue:

J F O Z X

Challenge 3: Whose turn is it?

Open the starter project, and navigate to Challenge 3’s playground page to begin.

Challenge 4: Reverse Queue

Navigate to Challenge 4’s playground page to begin.

Challenge 5: Double-ended Queue

A double-ended queue — a.k.a. a deque — is, as its name suggests, a queue where elements can be added or removed from the front or back.

Note:

In DoubleLinkedList.swift one additional property and function has been added:

Solutions

Solution to Challenge 1

Queues have a behavior of first-in-first-out. What comes in first must come out first. Items in the queue are inserted from the rear and removed from the front.

Solution to Challenge 2

Array

Keep in mind whenever the array is full, and you try to add a new element, a new array will be created with twice the capacity with existing elements being copied over.

T Y A R Z C Q O L Y S alnaioe (“J”) Q G U H M T O uqkaeoi (“E”) X U R H H O xevoaii () H I Q J S A Z oknuoii (“Z”) O B T G U G duvaeuo () F Z H E H juyeaii () D V C U R G omsoeio (“V”)

Linked list

S W I F T S W I F T R enqueue (“R”) W I F T O R C enqueue (“C”) I F T O R C dequeue () F T O R C dequeue () enqueue (K) F T O R C K S W I F T R O enqueue (“O”) dequeue () W I F T R O

Ring buffer

S W I F T r w S W I F T r w S W I F T r w S W I F T r w r w C W I F T r w C W I F T r w C W I F T r w C K I F T enqueue (“R”) return false enqueue (“O”) return false enqueue (“C”) return true enqueue (“K”) dequeue () dequeue () dequeue ()

Double stack

Left Stack S W I F T Right Stack R Left Stack S W I F T Right Stack enqueue (“R”)

Nekj Thays V C E T K B O Vidkz Fqirj X U Z D C T E Mupq Vvogz Qoxwj Zvoxx nexoeei () avxuoei (“I”)

Sugq Dresw A S R K E Z С С Pudmb Gxegb L E L V L U Cilv Zragv Gixzs Xtoyh mesiouo () itlauei (“B”)

Tagb Yjukw O Z V H С С Pilpl Sxunp P E M B I L Jukv Bluvb Kompl Wsujt ifniaoa (“Y”) niviuoa ()

Solution to Challenge 3

Creating a board game manager is straightforward. All you care about is whose turn it is. A queue data structure is the perfect choice to adopt the BoardGameManager protocol!

Solution to Challenge 4

A queue uses first-in-first-out, whereas a stack uses last-in-first-out. You can use a stack to help reverse the contents of a queue. By inserting all the contents of the queue into a stack, you reverse the order once you pop every single element off the stack!

Solution to Challenge 5

Deque is made up of common operations from the Queue and Stack data structures. There are many ways to implement a Deque. You could build one using a circular buffer, two stacks, an array, or a doubly linked list. The solution below makes use of a doubly linked list to construct a Deque.

8. Queues
10. Trees
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