Introduction

A linked list is a fundamental data structure which consists of Nodes that are connected to each other.

Other variations are:

• Double linked list
• Circular linked list (circular buffer)

The Singly Linked List

To visualize the data structure, if you want to store two integers 10 and 20 you will have a 2 node linked list that will have: [Node 1, Value 10] -> [Node 2, Value: 20] -> [null]

Node 1 points to Node 2 and Node 2 points to null.

The Linked List data structure permits the storage of data in an efficient manner.

Unlike arrays, the linked list data structure doesn’t pre-allocate space for the data, it grows and shrinks at runtime when needed.

The Linked List has many applications, but here are a few:

• Implementing undo-redo in a text editor.
• Representing a musical playlist.
• Managing a cache where entries are added and removed based on usage.

• Storing data in a system where memory is limited such as embedded systems.
• Buffering.

Depending of the implementation it can support operations such as:

• insert element at position
• append element
• delete last element
• delete element
• get first element
• get last element
• get element at position
• reverse list

Space and Time Complexity

The space and time complexity of the linked list operations depends on the implementation.

A basic implementation which stores the head and tail nodes has the following complexity:

Space Complexity is O(N), the list grows or shrinks linearly depending on how many elements are added at runtime.

The Time Complexity is differing for each operation:

• Append element to the end of the list takes O(1) time.
• Insert at position takes O(N) time.
• Delete at position takes O(N) time.
• Get first and last elements will take O(1) time.
• Get at position N will take O(N) time.

The operations that take O(N) time takes this much because you have to traverse the list’s for at least N nodes in order to perform it successfully. On the other hand, operations that take O(1) time do not require any traversals because the list holds pointers to the head (first Node) and tail (last Node).

Implementation in Kotlin

If you would like, feel free to stop reading the article here and start implementing a generic linked list in your favorite programming language. For me, it was a fun learning experience and I’ll hope it will be the same for you.

You’ll find the Kotlin implementation for the Linked List on the GitHub repo: Implementation | Tests.

Let’s review some:

Append an element at the end of the list

Append appends an element’s value at the end of the list. Since the method can access the head and tail of the list, the implementation is fairly straightforward.

If the list contains at least one element, use the tail to append the element then update the tail. If the list is empty, then add the new node to the head and tail.

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    fun append(value: T) {
        val newNode = Node(value, null)
        if (head == null) {
            head = newNode
            tail = newNode
        } else {
            tail!!.next = newNode
            tail = newNode
        }
        size += 1
    }

Get an element’s value from the list

To get an element from the list at position N the list has to be traversed until the position is reached.

A possible optimization here can be whether the traversal should start from the head or tail of the list.

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    fun get(position: Int): T? {
        var currentPosition = 0
        var currentNode = head
        while (currentPosition <= position && currentNode != null) {
            if (currentPosition == position) {
                return currentNode.value
            }
            currentNode = currentNode.next
            currentPosition += 1
        }
        return null
    }

Iterator Pattern

To iterate though the list in a Kotlin friendly fashion with linkedList.forEach {} the iterator pattern needs to be implemented.

Since the iterable is the Node<T> the list needs to implement the Iterable<Node<T>> interface, then implement the iterator function:

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    override fun iterator(): Iterator<Node<T>> {
        return NodeIterator(this)
    }

The NodeIterator is an inner class that implements the Iterator<Node<T>> interface and provides the iteration functionality.

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    class NodeIterator<T>(linkedList: LinkedList<T>) : Iterator<Node<T>> {
        private var currentNode: Node<T>? = linkedList.head

        override fun hasNext(): Boolean {
            return currentNode != null
        }

        override fun next(): Node<T> {
            val node = currentNode!!
            currentNode = currentNode?.next
            return node
        }
    }

listOf - An elegant constructor

Constructs a linked list from a variable number of arguments.

The listOf method is implemented with the help of an extension method:

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fun <T> LinkedList.Companion.listOf(vararg items: T): LinkedList<T> {
    val linkedList = LinkedList<T>()
    items.forEach {
        linkedList.append(it)
    }
    return linkedList
}

It provides the user of the Linked List API an elegant way to instantiate it: LinkedList.listOf(1, 2).

toArray - Transforms the Linked List ist into an array

The toArray is tricky to implement as an instance method. Since generic types are erased at runtime, the method doesn’t know what type T is.

By making this a generic extension method and defining a new reified type E the type of the generic method can be omitted when calling it. The compiler can infer it automatically.

For example, calling toArray() on a LinkedList<String>the type parameter E will be linkedList.toArray<String>().

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inline fun <reified E> LinkedList<E>.toArray(): Array<E?> {
    val newArray = arrayOfNulls<E>(this.size())
    this.forEachIndexed { i, node ->
        newArray[i] = node.value
    }
    return newArray
}