Swift Algorithm Club: Swift Linked List Data Structure

The Swift Algorithm Club is an open source project on implementing data structures and algorithms in Swift.

Every month, Kelvin Lau and I feature a cool data structure or algorithm from the club in a tutorial on this site. If you want to learn more about algorithms and data structures, follow along with us!

In this tutorial, you’ll learn how to implement a linked list in Swift 3. The linked list implementation was first implemented by Matthijs Hollemans, the founder of the Swift Algorithm Club.

Getting Started

A linked list is a sequence of data items, where each item is referred to as a node.

There are two main types of linked lists:

Singly linked lists, are linked lists where each node only has a reference to the next node.

Doubly linked lists, are linked lists where each node has a reference to the previous and next node.

You need to keep track of where the list begins and ends. That’s usually done with pointers called head and tail.

Linked List Implementation in Swift 3

In this section, you’ll implement a linked list in Swift 3.

Remember that a linked list is made up of nodes. So to start, let’s create a basic node class. Create a new Swift playground and add the following empty class:

public class Node {
 
}

Value

A node needs a value associated with it. Add the following between the curly braces:

var value: String
 
init(value: String) {
  self.value = value
}

You’ve declared a property named value of type String. In your own apps, this could be any datatype you want to store.

You also declare an initializer, which is required for initializing all non-optional stored properties for your class.

Next

In addition to a value, each node needs a pointer to the next node in the list.

To do this, add the following property to the class:

var next: Node?

You have declared a property named next of type Node. Note that you’ve made next an optional. This is because the last node in the linked list does not point to another node.

Previous

You are implementing a doubly-linked list so we also need a pointer to the previous node in the list.

To do this, add one last property to the class:

weak var previous: Node?

Note: To avoid ownership cycles, we declare the previous pointer to be weak. If you have a node A that is followed by node B in the list, then A points to B but also B points to A. In certain circumstances, this ownership cycle can cause nodes to be kept alive even after you deleted them. We don’t want that, so we make one of the pointers weak to break the cycle.

To learn more about ownership cycles, check out our ARC and Memory Management in Swift tutorial.

Linked List

Now that you have created the Node you also need to keep track of where the list begins and ends.

To do this, add this new LinkedList class to the bottom of the playground:

public class LinkedList {
  fileprivate var head: Node?
  private var tail: Node?

  public var isEmpty: Bool {
    return head == nil
  }

  public var first: Node? {
    return head
  }

  public var last: Node? {
    return tail
  }
}

This class will keep track of where the list begins and ends. It will also provide a number of other helper functions.

Append

To handle appending a new node on your list, you’ll declare a append(value:) method in your LinkedList class. Add the following new method to LinkedList:

public func append(value: String) {
  // 1
  let newNode = Node(value: value)
  // 2
  if let tailNode = tail {
    newNode.previous = tailNode
    tailNode.next = newNode
  } 
  // 3
  else {
    head = newNode
  }
  // 4
  tail = newNode
}

Let’s review this section by section:

• Create a new Node to contain the value. Remember, the purpose of the Node class is so that each item in the linked list can point to the previous and next node.
• If tailNode is not nil, that means there is something in the linked list already. If that’s the case, configure the new item to point to the tail of the list as it’s previous item. Similarly, configure the new last item on the list to point to the new node as it’s next item.
• Finally, set the tail of the list to be the new item in either case.

Printing Your Linked List

Let’s try out your new linked list. Outside the implementation of LinkedList, write the following into your playground:

let dogBreeds = LinkedList()
dogBreeds.append(value: "Labrador")
dogBreeds.append(value: "Bulldog")
dogBreeds.append(value: "Beagle")
dogBreeds.append(value: "Husky")

After defining the list, we will try print the list to the console:

print(dogBreeds)

You can bring up the console by pressing the following keys in combination: Command-Shift-Y. You should see the following printed out to the console:

LinkedList

That isn’t very helpful. To display a more readable output string, you can make LinkedList adopt the CustomStringConvertable protocol. To do this, add the following just below the implementation of your LinkedList class:

// 1
extension LinkedList: CustomStringConvertible {
  // 2
  public var description: String {
    // 3
    var text = "["
    var node = head
    // 4
    while node != nil {
      text += "\(node!.value)"
      node = node!.next
      if node != nil { text += ", " }
    }
    // 5
    return text + "]"
  }
}

Here’s how the code works:

1. You’ve declared an extension to your LinkedList class, and you’ve adopted the CustomStringConvertible protocol. This protocol expects you to implement a computed property with the name description, with the String type.
2. You’ve declared the description property. This is a computed property, a read only property that returns a String.
3. You’ve declared a text variable. This will hold the entire string. For now, it contains an opening brace to represent the start of the list.
4. You then loop through the list appending the value of each item to the text string.
5. You add a closing brace to the end of the text variable.

Now, when you call print on your LinkedList classes, you’ll get a nice representation of your list like this:

"[Labrador, Bulldog, Beagle, Husky]"