Swift Algorithm Club: Swift Linked List Data Structure

Chris Pilcher

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.

Note: New to the Swift Algorithm Club? Check out our getting started post first.

Getting Started

A linked list is a sequence of data item, 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.

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.

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 the print your `LinkedList` classes, you’ll get a nice representation of your list like this:

```"[Labrador, Bulldog, Beagle, Husky]"
```

Accessing Nodes

Even though a linked list works most efficiently when you move through nodes in order via previous and next, sometimes it is handy to access an item by index.

To do this, you will declare a `nodeAt(index:)` method in your `LinkedList` class. This will return the `Node` at the specified index.

Update the implementation of `LinkedList` to include the following:

 ```public func nodeAt(index: Int) -> Node? { // 1 if index >= 0 { var node = head var i = index // 2 while node != nil { if i == 0 { return node } i -= 1 node = node!.next } } // 3 return nil }```

Here’s what you’ve done:

1. Added a check that the specified `index` is not negative. This prevents an infinite loop if the `index` is a negative value.
2. Loop through the nodes until you reach the node at the specified `index` and return the node.
3. If the `index` less than 0 or greater than the number of items in the list, then return `nil`.

Removing All Nodes

Removing all nodes is simple. We just assign `nil` to the `head` and `tail`:

 ```public func removeAll() { head = nil tail = nil }```

Removing Individual Node

To remove an individual node, you will have to deal with three cases:

1. Removing the first node. The requires the `head` and `previous` pointers to be updated:
2. Removing a node in the middle of the list. This requires the `previous` and `next` pointers to be updated:
3. Removing the last node in the list. This requires the `next` and `tail` pointers to be updated:

Update the implementation of `LinkedList` to include:

 ```public func remove(node: Node) -> String { let prev = node.previous let next = node.next   if let prev = prev { prev.next = next // 1 } else { head = next // 2 } next?.previous = prev // 3   if next == nil { tail = prev // 4 }   // 5 node.previous = nil node.next = nil   // 6 return node.value }```

Here’s what you’ve done:

1. Update the `next` pointer if you are not removing the first node in the list.
2. Update the `head` pointer if you are removing the first node in the list.
3. Update the `previous` pointer to the `previous` pointer of the deleted node.
4. Update the `tail` if you are removing the last node in the list.
5. Assign `nil` to the removed nodes `previous` and `next` pointers.
6. Return the value for the removed node.

Generics

So far you’ve implemented a general-purpose linked list that stores `String` values. You’ve provided functionality to append, remove and access nodes in your `LinkedList` class. In this section we will use generics to abstract away the type requirement from our linked list.

Update the implementation of your `Node` class to the following:

 ```// 1 public class Node { // 2 var value: T var next: Node? weak var previous: Node?   // 3 init(value: T) { self.value = value } }```

Here’s what you’ve done:

1. You’ve changed the declaration of the `Node` class to take a generic type `T`.
2. Your goal is to allow the `Node` class to take in values of any type, so you’ll constrain your value property to be type `T` rather than a `String`.
3. You’ve also updated your initializer to take any type.

Generics: Challenge

Try updating the implementation of `LinkedList` to use generics.

The solution is provided in the spoiler section down below, but try it yourself first!

Solution Inside: Solution SelectShow

Your code should compile now, so let’s test this out! At the bottom of your playground file, add the following code to verify that your generic linked list is working:

 ```let dogBreeds = LinkedList() dogBreeds.append(value: "Labrador") dogBreeds.append(value: "Bulldog") dogBreeds.append(value: "Beagle") dogBreeds.append(value: "Husky")   let numbers = LinkedList() numbers.append(value: 5) numbers.append(value: 10) numbers.append(value: 15)```

Where To Go From Here?

I hope you enjoyed this tutorial on making a linked list!

Here is a Swift playground with the above code. You can also find alternative implementations and further discussion in the linked list section of the Swift Algorithm Club repository.

This was just one of the many algorithm clubs focused on the Swift Algorithm Club repository. If you’re interested in more, check out the repo.

If you have any questions on linked lists in Swift, please join the forum discussion below!

Note: The Swift Algorithm Club is always looking for more contributors. If you’ve got an interesting data structure, algorithm, or even an interview question to share, don’t hesitate to contribute! To learn more about the contribution process, check out our Join the Swift Algorithm Club article.
Chris Pilcher

I'm a software developer based in Auckland, New Zealand. I help maintain the Swift Algorithm Club.

Outside of coding, I enjoy video games and skydiving.

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