3. Linked List

Written by Matei Suica, Kelvin Lau and Vincent Ngo

A linked list is a collection of values arranged in a linear, unidirectional sequence. A linked list has several theoretical advantages over contiguous storage options such as the Kotlin Array or ArrayList:

• Constant time insertion and removal from the front of the list.
• Reliable performance characteristics.

As the diagram suggests, a linked list is a chain of nodes. Nodes have two responsibilities:

1. Hold a value.
2. Hold a reference to the next node. The absence of a reference to the next node, null, marks the end of the list.

In this chapter, you’ll implement a linked list and learn about the common operations associated with it. You’ll also learn about the time complexity of each operation. Open the starter project for this chapter so that you can dive right into the code.

Node

Create a new Kotlin file in src and name it Node.kt. Add the following to the file:

data class Node<T>(var value: T, var next: Node<T>? = null) {
  override fun toString(): String {
    return if (next != null) {
      "$value -> ${next.toString()}"
    } else {
      "$value"
    }
  }
}

Navigate to the Main.kt file and add the following inside main():

fun main() {
  "creating and linking nodes" example {
    val node1 = Node(value = 1)
    val node2 = Node(value = 2)
    val node3 = Node(value = 3)

    node1.next = node2
    node2.next = node3

    println(node1)
  }
}

You’ve just created three nodes and connected them:

Once you run Main.kt, you’ll see the following output in the console:

---Example of creating and linking nodes---
1 -> 2 -> 3

As far as practicality goes, this method of building lists is far from ideal. You can easily see that building long lists in this way is impractical. A common way to alleviate this problem is to build a LinkedList that manages the Node objects. You’ll do just that!

LinkedList

In src, create a new file and name it LinkedList.kt. Add the following to the file:

class LinkedList<T> {

  private var head: Node<T>? = null
  private var tail: Node<T>? = null
  private var size = 0

  fun isEmpty(): Boolean {
    return size == 0
  }

  override fun toString(): String {
    if (isEmpty()) {
      return "Empty list"
    } else {
      return head.toString()
    }
  }
}

A linked list has the concept of a head and tail, which refers to the first and last nodes of the list respectively:

You’ll also keep track of the size of the linked list in the size property. This might not seem useful yet, but it will come in handy later.

Adding values to the list

Next, you’re going to provide an interface to manage the Node objects. You’ll first take care of adding values. There are three ways to add values to a linked list, each having their own unique performance characteristics:

1. push: Adds a value at the front of the list.
2. append: Adds a value at the end of the list.
3. insert: Adds a value after a particular node of the list.

You’ll implement each of these in turn and analyze their performance characteristics.

push operations

Adding a value at the front of the list is known as a push operation. This is also known as head-first insertion. The code for it is deliciously simple.

Add the following method to LinkedList:

fun push(value: T) {
  head = Node(value = value, next = head)
  if (tail == null) {
    tail = head
  }
  size++
}

In the case in which you’re pushing into an empty list, the new node is both the head and tail of the list. Since the list now has a new node, you increment the value of size.

In Main.kt, add the following in main():

"push" example {
  val list = LinkedList<Int>()
  list.push(3)
  list.push(2)
  list.push(1)

  println(list)
}

Your console output will show this:

---Example of push---
1 -> 2 -> 3

This is pretty cool, but you can do even better. You’ll use the fluent interface pattern to chain multiple push calls. Go back to push() and add LinkedList<T> as its return type. Then, add a return this line at the end to return the list that you’ve just pushed an element into.

The method will now look like this:

fun push(value: T): LinkedList<T> {
  head = Node(value = value, next = head)
  if (tail == null) {
    tail = head
  }
  size++
  return this
}

In main(), you can now rewrite the previous example, making use of push()’s return value:

"fluent interface push" example {
  val list = LinkedList<Int>()
  list.push(3).push(2).push(1)
  println(list)
}

That’s more like it! Now that you can add multiple elements to the start of the list with ease.

append operations

The next operation you’ll look at is append. This adds a value at the end of the list, which is known as tail-end insertion.

In LinkedList.kt, add the following code just below push():

fun append(value: T) {
  // 1
  if (isEmpty()) {
    push(value)
    return
  }
  // 2
  tail?.next = Node(value = value)

  // 3
  tail = tail?.next
  size++
}

This code is relatively straightforward:

1. Like before, if the list is empty, you’ll need to update both head and tail to the new node. Since append on an empty list is functionally identical to push, you invoke push to do the work for you.
2. In all other cases, you create a new node after the current tail node. tail will never be null here because you’ve already handled the case where the list is empty in the if statement.
3. Since this is tail-end insertion, your new node is also the tail of the list.

Go back to Main.kt and write the following at the bottom of main():

"append" example {
  val list = LinkedList<Int>()
  list.append(1)
  list.append(2)
  list.append(3)

  println(list)
}

You’ll see the following output in the console:

---Example of append---
1 -> 2 -> 3

You can use the trick you learned for push() to get a fluid interface here too. It’s up to you if you’ve liked it or not but imagine how you could chain pushes and appends in a world of endless possibilities. Or just have some fun with it. :]

insert operations

The third and final operation for adding values is insert(afterNode: Node<T>). This operation inserts a value at a particular place in the list and requires two steps:

1. Finding a particular node in the list.
2. Inserting the new node after that node.

First, you’ll implement the code to find the node where you want to insert your value.

In LinkedList.kt, add the following code just below append:

fun nodeAt(index: Int): Node<T>? {
  // 1
  var currentNode = head
  var currentIndex = 0

  // 2
  while (currentNode != null && currentIndex < index) {
    currentNode = currentNode.next
    currentIndex++
  }

  return currentNode
}

nodeAt() tries to retrieve a node in the list based on the given index. Since you can only access the nodes of the list from the head node, you’ll have to make iterative traversals. Here’s the play-by-play:

1. You create a new reference to head and keep track of the current number of traversals.

2. Using a while loop, you move the reference down the list until you reach the desired index. Empty lists or out-of-bounds indexes will result in a null return value.

Now, you need to insert the new node.

Add the following method just below nodeAt():

fun insert(value: T, afterNode: Node<T>): Node<T> {
  // 1
  if (tail == afterNode) {
    append(value)
    return tail!!
  }
  // 2
  val newNode = Node(value = value, next = afterNode.next)
  // 3
  afterNode.next = newNode
  size++
  return newNode
}

Here’s what you’ve done:

1. In the case where this method is called with the tail node, you call the functionally equivalent append method. This takes care of updating tail.
2. Otherwise, you create a new node and link its next property to the next node of the list.
3. You reassign the next value of the specified node to link it to the new node that you just created.

To test things, go back to Main.kt and add the following to the bottom of main():

"inserting at a particular index" example {
  val list = LinkedList<Int>()
  list.push(3)
  list.push(2)
  list.push(1)

  println("Before inserting: $list")
  var middleNode = list.nodeAt(1)!!
  for (i in 1..3) {
    middleNode = list.insert(-1 * i, middleNode)
  }
  println("After inserting: $list")
}

You’ll see the following output:

---Example of inserting at a particular index---
Before inserting: 1 -> 2 -> 3
After inserting: 1 -> 2 -> -1 -> -2 -> -3 -> 3

Performance analysis

Whew! You made good progress so far. To recap, you’ve implemented the three operations that add values to a linked list and a method to find a node at a particular index.

Next, you’ll focus on the opposite action: removal operations.

Removing values from the list

There are three primary operations for removing nodes:

1. pop: Removes the value at the front of the list.
2. removeLast: Removes the value at the end of the list.
3. removeAfter: Removes a value anywhere in the list.

You’ll implement all three and analyze their performance characteristics.

pop operations

Removing a value at the front of the list is often referred to as pop. This operation is almost as simple as push(), so dive right in.

Add the following method to LinkedList:

fun pop(): T? {
  if (!isEmpty()) size--
  val result = head?.value
  head = head?.next
  if (isEmpty()) {
    tail = null
  }

  return result
}

pop() returns the value that was removed from the list. This value is optional since it’s possible that the list is empty.

By moving the head down a node, you’ve effectively removed the first node of the list. The garbage collector will remove the old node from memory once the method finishes since there will be no more references attached to it. If the list becomes empty, you set tail to null as well.

To test, go to Main.kt and add the following code at the bottom inside main():

"pop" example {
  val list = LinkedList<Int>()
  list.push(3)
  list.push(2)
  list.push(1)

  println("Before popping list: $list")
  val poppedValue = list.pop()
  println("After popping list: $list")
  println("Popped value: $poppedValue")
}

You’ll see the following output:

---Example of pop---
Before popping list: 1 -> 2 -> 3
After popping list: 2 -> 3
Popped value: 1

removeLast operations

Removing the last node of the list is somewhat inconvenient.

Although you have a reference to the tail node, you can’t chop it off without having a reference to the node before it. Thus, you need to traverse the whole list to find the node before the last.

Add the following code just below pop():

fun removeLast(): T? {
  // 1
  val head = head ?: return null
  // 2
  if (head.next == null) return pop()
  // 3
  size--

  // 4
  var prev = head
  var current = head

  var next = current.next
  while (next != null) {
    prev = current
    current = next
    next = current.next
  }
  // 5
  prev.next = null
  tail = prev
  return current.value
}

Here’s what’s happening:

1. If head is null, there’s nothing to remove, so you return null.
2. If the list only consists of one node, removeLast is functionally equivalent to pop. Since pop will handle updating the head and tail references, you can delegate this work to the pop function.
3. At this point, you know that you’ll be removing a node, so you update the size of the list accordingly.
4. You keep searching for the next node until current.next is null. This signifies that current is the last node of the list.
5. Since current is the last node, you disconnect it using the prev.next reference. You also make sure to update the tail reference.

Go back to Main.kt, and in main(), add the following to the bottom:

"removing the last node" example {
  val list = LinkedList<Int>()
  list.push(3)
  list.push(2)
  list.push(1)

  println("Before removing last node: $list")
  val removedValue = list.removeLast()

  println("After removing last node: $list")
  println("Removed value: $removedValue")
}

You’ll see the following at the bottom of the console:

---Example of removing the last node---
Before removing last node: 1 -> 2 -> 3
After removing last node: 1 -> 2
Removed value: 3

removeLast() requires you to traverse down the list. This makes for an O(n) operation, which is relatively expensive.

Remove operations

The final remove operation is removing a node at a particular point in the list. This is achieved much like insert(). You’ll first find the node immediately before the node you wish to remove and then unlink it.

Navigate back to LinkedList.kt and add the following method below removeLast():

fun removeAfter(node: Node<T>): T? {
  val result = node.next?.value

  if (node.next == tail) {
    tail = node
  }

  if (node.next != null) {
    size--
  }

  node.next = node.next?.next
  return result
}

Special care needs to be taken if the removed node is the tail node since the tail reference will need to be updated.

Now, add the following example to main() to test removeAfter(). You know the drill:

"removing a node after a particular node" example {
  val list = LinkedList<Int>()
  list.push(3)
  list.push(2)
  list.push(1)

  println("Before removing at particular index: $list")
  val index = 1
  val node = list.nodeAt(index - 1)!!
  val removedValue = list.removeAfter(node)

  println("After removing at index $index: $list")
  println("Removed value: $removedValue")
}

You’ll see the following output in the console:

---Example of removing a node after a particular node---
Before removing at particular index: 1 -> 2 -> 3
After removing at index 1: 1 -> 3
Removed value: 2

Try adding more elements and play around with the value of the index. Similar to insert(), the time complexity of this operation is O(1), but it requires you to have a reference to a particular node beforehand.

Performance analysis

You’ve hit another checkpoint. To recap, you’ve implemented the three operations that remove values from a linked list:

At this point, you’ve defined an interface for a linked list that most programmers around the world can relate to. However, there’s work to be done to adorn the Kotlin semantics. In the next half of the chapter, you’ll focus on making the interface better by bringing it closer to idiomatic Kotlin.

Kotlin collection interfaces

The Kotlin Standard Library has a set of interfaces that help define what’s expected of a particular type. Each of these interfaces provides certain guarantees on characteristics and performance. Of these set of interfaces, four are referred to as collection interfaces.

Here’s a small sample of what each interface represents:

• Tier 1, Iterable: An iterable type provides sequential access to its elements via an Iterator.

• Tier 2, Collection: A collection type is an iterable type that provides additional functionality, allowing you to check if the collection contains a particular element or a collection of elements.

• Tier 3, MutableIterable: Provides a MutableIterator, which allows items to be removed from the given collection.

• Tier 4, MutableCollection: Unlike a simple Collection, a MutableCollection interface provides methods to alter the collection. For example, you can add and remove elements, and even clear the entire collection.

There’s more to be said for each of these. You’ll learn more about each of them when you need to conform to them.

A linked list can get to the tier 4 of the collection interfaces. Since a linked list is a chain of nodes, adopting the Iterable interface makes sense. And because you’ve already implemented adding elements and removing them, it’s pretty clear we can go all the way to the MutableCollection interface.

Becoming a Kotlin mutable collection

In this section, you’ll look into implementing the MutableCollection interface. A mutable collection type is a finite sequence and provides nondestructive sequential access but also allows you to modify the collection.

Iterating through elements

Reaching Tier 1 means implementing Iterable in the LinkedList. To make things easier, first make reading the size available outside the class. Modify the size property in LinkedList.kt, so that the property itself is public, but its setter remains private:

var size = 0
  private set

Then, add the Iterable interface to the class definition. The definition will now look like:

class LinkedList<T> : Iterable<T> {
  ...
}

This means that you’re now required to add the following method to fulfill the Iterable interface:

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

Right now, the compiler complains because it doesn’t know what a LinkedListIterator is. Create a class named LinkedListIterator and make it implement the Iterator interface:

class LinkedListIterator<T> : Iterator<T> {
  override fun next(): T {
    TODO("not implemented")
  }
  override fun hasNext(): Boolean {
    TODO("not implemented")
  }
}

To iterate through the linked list, you need to have a reference to the list. Add this parameter to the constructor:

class LinkedListIterator<T>(
  private val list: LinkedList<T>
) : Iterator<T> {
  ...
}

You can now start implementing the required methods, starting with the easier one, hasNext(). This method indicates whether your Iterable still has values to read. You’ll need to keep track of the position that the iterator has in the collection, so create an index field:

private var index = 0

Then, you can easily check if the position you’re at is less than the total number of nodes:

override fun hasNext(): Boolean {
  return index < list.size
}

For next(), the one that reads the values of the nodes, you can use another property to help you out. You’ll want to keep track of the last node, so you can easily find the next one:

private var lastNode: Node<T>? = null

The next() function looks like:

override fun next(): T {
  // 1
  if (index >= list.size) throw IndexOutOfBoundsException()
  // 2
  lastNode = if (index == 0) {
    list.nodeAt(0)
  } else {
    lastNode?.next
  }
  // 3
  index++
  return lastNode!!.value
}

Here are the crucial bits of this function, step-by-step:

1. You check that there are still elements to return. If there aren’t, you throw an exception. This should never be the case if clients use the Iterator correctly, always checking with hasNext() before trying to read a value from it with next().
2. If this is the first iteration, there is no lastNode set, so you take the first node of the list. After the first iteration, you can get the next property of the last node to find the next one.
3. Since the lastNode property was updated, you need to update the index too. You’ve now gone through one more iteration, so the index increments.

Now that you’ve implemented Iterator, you can do some really cool things. For example, you can iterate your linked list with a regular Kotlin for loop and print the double of each element in a list.

Add this to main():

"printing doubles" example {
  val list = LinkedList<Int>()
  list.push(3)
  list.push(2)
  list.push(1)
  println(list)

  for (item in list) {
    println("Double: ${item * 2}")
  }
}

The output you’ll get is:

---Example of printing doubles---
1 -> 2 -> 3
Double: 2
Double: 4
Double: 6

Cool, huh? A for loop helps you a lot but there are still things to implement if you want to have a fully featured MutableCollection.

Becoming a collection

Being a Collection requires more than just being an Iterable class. Change the definition of your LinkedList to implement Collection:

class LinkedList<T> : Iterable<T>, Collection<T>  {
  ...
}

Of course, you could now remove Iterable because a Collection is an Iterable anyway. You may also leave it there so that you can see the progress you’re making.

The compiler will start complaining about the methods you need to implement. Here are some quick wins: You already have isEmpty() and size, so you can add the override keyword in front of them.

override var size = 0
  private set

override fun isEmpty(): Boolean {
  return size == 0
}

You still need to implement two more methods, but the good news is that you can use one to implement the other easily:

override fun contains(element: T): Boolean {
  TODO("not implemented")
}

override fun containsAll(elements: Collection<T>): Boolean {
  TODO("not implemented")
}

Since you can now iterate through the list with for, the implementation of contains is straightforward:

override fun contains(element: T): Boolean {
  for (item in this) {
    if (item == element) return true
  }
  return false
}

This method iterates through all elements of the list if needed, so it has a complexity of O(n).

The second method is similar; it just works with a collection of elements.

override fun containsAll(elements: Collection<T>): Boolean {
  for (searched in elements) {
    if (!contains(searched)) return false
  }
  return true
}

As you’d probably guess, this is an inefficient method, it’s O(n^2). But if the Collection interface requires it, you need to provide it.

Mutating while iterating

To get to the 3rd tier, you need to make LinkedList a MutableIterable. If you add this interface to the list that LinkedList implements, you’ll see the compiler complain again because iterator() doesn’t return a MutableIterator. This time, start the other way around. Make LinkedListIterator implement the MutableIterator<T> interface:

class LinkedListIterator<T>(
  private val list: LinkedList<T>
) : Iterator<T>, MutableIterator<T> {
  ...
}

Again, MutableIterator is a broader interface than Iterator, so you can remove Iterator from the list of implemented interfaces.

You’ll need to add the remove() method to comply with the new interface you’ve added:

override fun remove() {
  // 1
  if (index == 1) {
    list.pop()
  } else {
    // 2
    val prevNode = list.nodeAt(index - 2) ?: return
    // 3
    list.removeAfter(prevNode)
    lastNode = prevNode
  }
  index--
}

Here’s a breakdown of how this code uses the methods LinkedList already has:

1. The simplest case is when you’re at the beginning of the list. Using pop() will do the trick.
2. If the iterator is somewhere inside the list, it needs to find the previous node. That’s the only way to remove items from a linked list.
3. The iterator needs to step back so that next() returns the correct method the next time. This means reassigning the lastNode and decreasing the index.

Now, go to LinkedList.kt and add MutableIterable to the class definition:

class LinkedList<T>: Iterable<T>, Collection<T>, MutableIterable<T>  {
  ...
}

Modify iterator() to return a MutableIterator, which LinkedListIterator implements:

override fun iterator(): MutableIterator<T> {
  return LinkedListIterator(this)
}

Final step: Mutable collection

You’ve already completed the hardest steps, so this last step shouldn’t be too bad. First, add the MutableCollection interface to the definition of LinkedList:

class LinkedList<T>: Iterable<T>, Collection<T>, MutableIterable<T>, MutableCollection<T>  {
  ...
}

This will make you add six more methods:

override fun add(element: T): Boolean {
  TODO("not implemented")
}

override fun addAll(elements: Collection<T>): Boolean {
  TODO("not implemented")
}

override fun clear() {
  TODO("not implemented")
}

override fun remove(element: T): Boolean {
  TODO("not implemented")
}

override fun removeAll(elements: Collection<T>): Boolean {
  TODO("not implemented")
}

override fun retainAll(elements: Collection<T>): Boolean {
  TODO("not implemented")
}

Three of them are relatively simple to implement. add(), addAll() and clear() are almost one-liners:

override fun add(element: T): Boolean {
  append(element)
  return true
}

override fun addAll(elements: Collection<T>): Boolean {
  for (element in elements) {
    append(element)
  }
  return true
}

override fun clear() {
  head = null
  tail = null
  size = 0
}

Since the LinkedList doesn’t have a fixed size, add() and addAll() are always successful and need to return true. For the removal of elements, you’ll use a different approach for iterating through your MutableIterable linked list. This way, you can benefit from your MutableIterator:

override fun remove(element: T): Boolean {
  // 1
  val iterator = iterator()
  // 2
  while (iterator.hasNext()) {
    val item = iterator.next()
    // 3
    if (item == element) {
      iterator.remove()
      return true
    }
  }
  // 4
  return false
}

This method is a little complex, so here’s the step-by-step walkthrough:

1. Get an iterator that will help you iterate through the collection.
2. Create a loop that checks if there are any elements left, and gets the next one.
3. Check if the current element is the one you’re looking for, and if it is, remove it.
4. Return a boolean that signals if an element has been removed.

With removeAll(), you can make use of remove():

override fun removeAll(elements: Collection<T>): Boolean {
  var result = false
  for (item in elements) {
    result = remove(item) || result
  }
  return result
}

The return value of removeAll is true if any elements were removed.

The last method to implement is retainAll(), which should remove any elements in the list besides the ones passed in as the parameter. You’ll need to approach this the other way around. Iterate through your list once and remove any element that is not in the parameter. Luckily, the parameter of retainAll is also a collection, so you can use all of the methods you implemented yourself, like contains:

override fun retainAll(elements: Collection<T>): Boolean {
  var result = false
  val iterator = this.iterator()
  while (iterator.hasNext()) {
    val item = iterator.next()
    if (!elements.contains(item)) {
      iterator.remove()
      result = true
    }
  }
  return result
}

Congrats! You finished the implementation, so it’s time for some testing. Go back into Main.kt and add these at the end of main():

"removing elements" example {
  val list: MutableCollection<Int> = LinkedList()
  list.add(3)
  list.add(2)
  list.add(1)

  println(list)
  list.remove(1)
  println(list)
}

"retaining elements" example {
  val list: MutableCollection<Int> = LinkedList()
  list.add(3)
  list.add(2)
  list.add(1)
  list.add(4)
  list.add(5)

  println(list)
  list.retainAll(listOf(3, 4, 5))
  println(list)
}

"remove all elements" example {
  val list: MutableCollection<Int> = LinkedList()
  list.add(3)
  list.add(2)
  list.add(1)
  list.add(4)
  list.add(5)

  println(list)
  list.removeAll(listOf(3, 4, 5))
  println(list)
}

As you’ll see momentarily, your first challenge in this chapter is to check the output of each example to make sure it’s correct. You’ll also see the rest of the challenges after a quick summary.

Challenges

In these challenges, you’ll work through five common scenarios for the linked list. These problems are relatively easy compared to most challenges, and they will serve to solidify your knowledge of data structures. You’ll find the solutions to the challenges at the end of this chapter.

Challenge 1: Reverse a linked list

Create an extension function that prints out the elements of a linked list in reverse order. Given a linked list, print the nodes in reverse order. For example:

1 -> 2 -> 3

// should print out the following:
3
2
1

Solution 1

A straightforward way to solve this problem is to use recursion. Since recursion allows you to build a call stack, you need to call the print statements as the call stack unwinds.

Your first task is to define an extension function for LinkedList. Add the following helper function to your solution file:

fun <T> LinkedList<T>.printInReverse() {
  this.nodeAt(0)?.printInReverse()
}

This function forwards the call to the recursive function that traverses the list, node by node. To traverse the list, add this extension function for Node:

fun <T> Node<T>.printInReverse() {
  this.next?.printInReverse()
}

As you’d expect, this function calls itself on the next node. The terminating condition is somewhat hidden in the null-safety operator. If the value of next is null, the function stops because there’s no next node on which to call printInReverse(). You’re almost done; the next step is printing the nodes.

Printing

Where you add the print statement will determine whether you print the list in reverse order or not. Update the function to the following:

fun <T> Node<T>.printInReverse() {
  this.next?.printInReverse()
  // 1
  if (this.next != null) {
    print(" -> ")
  }
  // 2
  print(this.value.toString())
}

Any code that comes after the recursive call is called only after the base case triggers (i.e., after the recursive function hits the end of the list).

1. First, you check if you’ve reached the end of the list. That’s the beginning of the reverse printing, and you’ll not add an arrow there. The arrows start with the second element of the reverse output. This is just for pretty formatting.

2. As the recursive statements unravel, the node data gets printed.

Test it out!

Write the following at the bottom of main():

"print in reverse" example {
  val list = LinkedList<Int>()
  list.add(3)
  list.add(2)
  list.add(1)
  list.add(4)
  list.add(5)

  println(list)
  list.printInReverse()
}

You’ll see the following output:

---Example of print in reverse---
3 -> 2 -> 1 -> 4 -> 5
5 -> 4 -> 1 -> 2 -> 3

The time complexity of this algorithm is O(n) since you have to traverse each node of the list.

Challenge 2: The item in the middle

Given a linked list, find the middle node of the list. For example:

1 -> 2 -> 3 -> 4
// middle is 3

1 -> 2 -> 3
// middle is 2

Solution 2

One solution is to have two references traverse down the nodes of the list where one is twice as fast as the other. Once the faster reference reaches the end, the slower reference will be in the middle. Write the following function:

fun <T> LinkedList<T>.getMiddle(): Node<T>? {
  var slow = this.nodeAt(0)
  var fast = this.nodeAt(0)

  while (fast != null) {
    fast = fast.next
    if (fast != null) {
      fast = fast.next
      slow = slow?.next
    }
  }

  return slow
}

In the while declaration, you bind the next node to fast. If there’s a next node, you update fast to the next node of fast, effectively traversing down the list twice. slow is updated only once. This is also known as the runner technique.

Try it out!

Write the following at the bottom of Main.kt:

"print middle" example {
  val list = LinkedList<Int>()
  list.add(3)
  list.add(2)
  list.add(1)
  list.add(4)
  list.add(5)

  println(list)
  println(list.getMiddle()?.value)
}

You’ll see the following output:

---Example of print middle---
3 -> 2 -> 1 -> 4 -> 5
1

The time complexity of this algorithm is O(n) since you traversed the list in a single pass. The runner technique helps solve a variety of problems associated with the linked list.

Challenge 3: Reverse a linked list

To reverse a list is to manipulate the nodes so that the nodes of the list are linked in the opposite direction. For example:

// before
1 -> 2 -> 3

// after
3 -> 2 -> 1

Solution 3

To reverse a linked list, you need to visit each node and update the next reference to point in the other direction. This can be a tricky task since you’ll need to manage multiple references to multiple nodes. To do this, you would also need access to the head and tail of your liked list. Since you’re implementing an extension function, you won’t have access to these variables. Luckily, there’s a simpler solution that has a small drawback discussed later.

You can easily reverse a list by using a recursive function that goes to the end of the list and then starts copying the nodes when it returns, in a new linked list. Here’s how this function would look like:

private fun <T> addInReverse(list: LinkedList<T>, node: Node<T>) {
  // 1
  val next = node.next
  if (next != null) {
    // 2
    addInReverse(list, next)
  }
  // 3
  list.append(node.value)
}

The following explains how this function works:

1. Get the next node of the list, starting from the one you’ve received as a parameter.

2. If there’s a following node, recursively call the same function; however, now the starting node is the one after the current node.

3. When you reach the end, start adding the nodes in the reverse order.

O(n) time complexity, short and sweet! The only drawback is that you need a new list, which means that the space complexity is also O(n).

To use this helper function conveniently on a LinkedList, add this extension function:

fun <T> LinkedList<T>.reversed(): LinkedList<T> {
  val result = LinkedList<T>()
  val head = this.nodeAt(0)
  if (head != null) {
    addInReverse(result, head)
  }
  return result
}

This extension creates a new LinkedList and fills it with nodes by calling addInReverse(), passing in the first node of the current list.

Try it out!

Test reversed() by writing the following at the bottom of main():

"reverse list" example {
  val list = LinkedList<Int>()
  list.add(3)
  list.add(2)
  list.add(1)
  list.add(4)
  list.add(5)

  println("Original: $list")
  println("Reversed: ${list.reversed()}")
}

You’ll see the following output:

---Example of reverse list---
Original: 3 -> 2 -> 1 -> 4 -> 5
Reversed: 5 -> 4 -> 1 -> 2 -> 3

Challenge 4: Merging two linked lists

Create a function that takes two sorted linked lists and merges them into a single sorted linked list

Your goal is to return a new linked list that contains the nodes from two lists in sorted order. You may assume they are both sorted in ascending order. For example:

// list1
1 -> 4 -> 10 -> 11

// list2
-1 -> 2 -> 3 -> 6

// merged list
-1 -> 1 -> 2 -> 3 -> 4 -> 6 -> 10 -> 11

Solution 4

The solution to this problem is to continuously pluck nodes from the two sorted lists and add them to a new list. Since the two lists are sorted, you can compare the next node of both lists to see which one should be the next one to add to the new list.

Setting up

You’ll begin by checking the cases where one or both of the lists are empty. Create the following mergeSorted extension function:

fun <T : Comparable<T>> LinkedList<T>.mergeSorted(
  otherList: LinkedList<T>
): LinkedList<T> {
  if (this.isEmpty()) return otherList
  if (otherList.isEmpty()) return this

  val result = LinkedList<T>()

  return result
}

If one is empty, you return the other. You also introduce a new reference to hold a new LinkedList. The strategy is to merge the nodes in this and otherList into result in sorted order.

Next, you need to write a helper function that adds the current node to the result list and returns the next node. You’ll use this function in your algorithm multiple times, so it’s useful to have it extracted:

private fun <T : Comparable<T>> append(
  result: LinkedList<T>,
  node: Node<T>
): Node<T>? {
  result.append(node.value)
  return node.next
}

Merging

Add the following to mergeSorted immediately below the declaration for result and right above return result:

// 1
var left = nodeAt(0)
var right = otherList.nodeAt(0)
// 2
while (left != null && right != null) {
  // 3
  if (left.value < right.value) {
    left = append(result, left)
  } else {
    right = append(result, right)
  }
}

Here’s how it works:

1. You start with the first node of each list.
2. The while loop continues until one of the lists reaches its end.
3. You compare the first nodes left and right to append to the result.

Since this loop depends on both left and right, it will terminate even if there are nodes left in either list.

Finally, add the following below the newly added code, and above return result, to handle the remaining nodes:

while (left != null) {
  left = append(result, left)
}

while (right != null) {
  right = append(result, right)
}

Try it out!

Write the following at the bottom of main():

"merge lists" example {
  val list = LinkedList<Int>()
  list.add(1)
  list.add(2)
  list.add(3)
  list.add(4)
  list.add(5)

  val other = LinkedList<Int>()
  other.add(-1)
  other.add(0)
  other.add(2)
  other.add(2)
  other.add(7)

  println("Left: $list")
  println("Right: $other")
  println("Merged: ${list.mergeSorted(other)}")
}

You’ll see the following output:

---Example of merge lists---
Left: 1 -> 2 -> 3 -> 4 -> 5
Right: -1 -> 0 -> 2 -> 2 -> 7
Merged: -1 -> 0 -> 1 -> 2 -> 2 -> 2 -> 3 -> 4 -> 5 -> 7

This algorithm has a time complexity of O(m + n), where m is the # of nodes in the first list, and n is the # of nodes in the second list.

Key points

• Linked lists are linear and unidirectional. As soon as you move a reference from one node to another, you can’t go back.
• Linked lists have a O(1) time complexity for head-first insertions. Arrays have O(n) time complexity for head-first insertions.
• Conforming to Kotlin collection interfaces, such as Iterable and Collection, offers a host of helpful methods for a reasonably small amount of requirements.