Introduction to Linked Lists with Scala | CodeSignal Learn

Introduction to Linked Lists with Scala | CodeSignal Learn

Introduction to Linked Lists

Hello there! Today, we will explore Linked Lists, a core data structure crucial for organized data management and establishing relationships between data. We will mark some essential milestones: an introduction to Linked Lists, their real-world applications, their implementation in Scala, and the different operations you can perform on them. By the end of this lesson, you will be well-equipped to implement and operate Linked Lists using Scala. Let's get started!

Understanding the Concept

A Linked List is a linear data structure similar to arrays. However, unlike arrays, they are not stored in contiguous memory locations. Each element in a Linked List is part of a node . A node comprises data and a reference (or link) to the next node in the sequence. This structure facilitates efficient insertions and deletions. The head is also an essential concept in Linked Lists. It is the first node in the list and serves as a reference to the entire list. An empty Linked List can be represented by a null reference or, more safely in Scala, by None when using Option[Node] . Singly linked lists often come up in coding interviews. Interviewers from tech giants, start-ups, and just about every company testing your coding abilities will pose challenges based on this concept. Singly linked lists are quite common in real-world applications. In fact, Scala's immutable List — one of the most widely used collections in Scala projects — is implemented as a singly linked list. They form excellent foundational knowledge for understanding doubly linked lists, which are chosen when bidirectional traversal is needed. A singly linked list contains nodes with a single link pointing to the next node, whereas a doubly linked list has nodes with links to both the next and the previous nodes.

Implementing Linked Lists - Creating Node

To begin implementing Linked Lists, we first need to understand the structure of a node, the building block of a Linked List. In Scala, we'll use a case class to serve as a blueprint for a node . A Node class mainly consists of data (the data you want to store) and next (the reference to the next node). In our case, we'll create a case class Node to store integer data, with a default value of None for the next node. Scala case class Node(var data: Int, var next: Option[Node] = None) Fantastic! You now know how to set up a node in a Linked List.

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Implementing Linked Lists - Append Method

In this section, we'll learn how to add a new node at the end of our Linked List. We'll implement an append method in our LinkedList class for this. class LinkedList {private var head: Option[Node] = None def append(data: Int): Unit = {val node = Node(data) head match {case None => head = Some(node) case Some(h) => def findLast(current: Node): Node = current.next match {case None => current case Some(nextNode) => findLast(nextNode)} val last = findLast(h) last.next = Some(node)}}} This code checks if head is None, indicating an empty list, and sets head to the new node. If the list is not empty, it uses a helper function to traverse to the end of the list and appends the new node.

Implementing Linked Lists - AddFirst Method

Now, what if we want to add a new node at the beginning of our list? We'll write a method addFirst to achieve this operation. def addFirst(data: Int): Unit = {val node = Node(data, head) head = Some(node)} It reassigns the head to point to the new node, which now links to the former head.

Implementing Linked Lists - Delete Method

Removing a node from a Linked List is also an essential functionality. We will add a delete method to our LinkedList class to remove a node with a particular data value. def delete(data: Int): Unit = {head match {case None => // Do nothing case Some(h) if h.data == data => head = h.next case Some(h) => def deleteFrom(node: Node): Unit = node.next match {case Some(nextNode) if nextNode.data == data => node.next = nextNode.next case Some(nextNode) => deleteFrom(nextNode) case None => ()} deleteFrom(h)}} We traverse the list similarly to the append method, searching for a node with the specified data, and remove it by updating the link of the previous node.

Scala

class LinkedList {
  private var head: Option[Node] = None

  def append(data: Int): Unit = {
    val node = Node(data)

    head match {
      case None => head = Some(node)
      case Some(h) =>
        def findLast(current: Node): Node =
          current.next match {
            case None => current
            case Some(nextNode) => findLast(nextNode)
          }

        val last = findLast(h)
        last.next = Some(node)
    }
  }
}

Scala

def delete(data: Int): Unit = {
  head match {
    case None => // Do nothing
    case Some(h) if h.data == data =>
      head = h.next
    case Some(h) =>
      def deleteFrom(node: Node): Unit =
        node.next match {
          case Some(nextNode) if nextNode.data == data =>
            node.next = nextNode.next
          case Some(nextNode) =>
            deleteFrom(nextNode)
          case None => ()
        }

      deleteFrom(h)
  }
}