Implementing a Linked List

A linked list is a popular data structure used in computer science and programming for sorting and managing collections of data.

Implementing a linked list in your code requires a bit more than simply declaring like you would with an array. You'll need to start by creating a Node class to represent individual nodes in the list as well as a LinkedList class to manage nodes and their connections. In this article, we'll explore how to write these classes in JavaScript, and how to use them to add, insert, remove, and various other functions that could be useful with a linked list. By the end of this article, you'll have a solid understanding of how to implement and use linked lists in your own code.

Creating the classes

As mentioned above, you need to create a Node class that represents the individual nodes in the linked list. The Node class has two properties:

data: represents the value stored in the node
next: represents a reference to the next node in the linked list.

The constructor function takes a data parameter, which is used to initialize the data property, and sets the next property to null by default.

// Define the class for a Node
class Node {
  constructor(data) {
    this.data = data;
    this.next = null;
  }
}

Next is to define the LinkedList that manages the nodes and their connections. The LinkedList class has 3 properties:

head: represents the first node
tail: represents the last node
size: represents the number of nodes present

By default, all 3 properties are set to 0 or null.

// Define the class for the Linked List
class LinkedList {
  constructor() {
    this.head = null;
    this.tail = null;
    this.size = 0;
  }
}

With these classes defined, you can start using them to create and manipulate linked lists in your code. For example, you can create a new linked list like this:

const myList = new LinkedList();

The common functions

Some common functions can be implemented when using a linked list:

add(data): add an element to the end of the linked list
insert(data, position: Insert an element at a specific position/
remove(data): Remove the first occurrence of an element.
indexOf(data): Find the position of the first occurrence of an element.
isEmpty(): Check if the linked list is empty.
size(): Return the number of elements in the linked list.
getHead(): Return the head node.
getTail(): Return the tail node.
clear(): Remove all elements in the linked list.

As you read on, you will learn more about these functions and how they are implemented in code. By understanding how these functions work, you can better manipulate and manage the data stored within a linked list.

Adding a node to the end

The function will take the value of the new node being added to the linked list.
A new Node object is created with the data value passed into the function.
If the linked list is empty (this.head is null), the new node becomes the first and only node in the list. Both this.head and this.tail are set to the new node.
If the linked list is not empty, the next property of the current tail node is set to the new node, and this.tail is updated to the point to the new node, effectively making it the new tail of the list.
The size property of LinkedList is incremented to reflect the addition of the new node.

add(data) {
    const newNode = new Node(data);

    if (!this.head) {
      this.head = newNode;
    } else {
      this.tail.next = newNode;
    }

    this.tail = newNode;
    this.size++;
  }

Insert a node at a specific position

The function takes two parameters: data, the value of the new node being inserted, and position, which is the index at which the new node should be inserted.
If the position is less than 0 or greater than the size of the linked list, the function returns false, indicating that the inspection was not successful.
A new Node object is created with the data value passed into the function.
If position is equal to the size of the linked list, the new node becomes the tail of the linked list. The next property of the current tail node is set to the new node, and this.tail is updated to point to the new node.
If position is 0, the new node becomes the head of the linked list. The next property of the new node is set to the current head node, and this.head, is then updated to point to the new node.
If position is somewhere in the middle of the linked list, the function uses a loop to find the node that comes before the position where the new node should be inserted (previous) and the node that comes after the position (current).
The next property of the new node is set to current, and the next property of previous is set to the new node. This effectively inserts the new node between previous and current.
The size property of the LinkedList object is incremented to reflect the addition of the new node.
The function returns true to indicate that the insertion was successful.

insert(data, position) {
    if (position < 0 || position > this.size) {
      return false;
    }

    const newNode = new Node(data);

    if (position === 0) {
      newNode.next = this.head;
      this.head = newNode;
    } else if (position === this.size) {
      this.tail.next = newNode;
      this.tail = newNode;
    } else {
      let current = this.head;
      let previous = null;
      let index = 0;

      while (index < position) {
        previous = current;
        current = current.next;
        index++;
      }

      newNode.next = current;
      previous.next = newNode;
    }

    this.size++;
    return true;
  }

Remove a node from the linked list

Declare variables current and previous. These are used to keep track of the current and previous nodes in the linked list as the method iterates through the list to find the node to remove. Current is set to the head of the linked list, and previous is set to null.
Loop through the linked list while current is not null.
If the data of current is equal to the data parameter passed into the function, the node to be removed has been found.
1. Check if previous is null. This will determine if the node to be removed is the head of the linked list. If it is, the head is set to the current node's next property.
2. If previous is not null, we need to update the previous.next to current.next. This effectively skips over the current node and points directly to the node that comes after it, removing it from the linked list.
3. Decrement the size property of the linked list.
4. Return data of the removed node (when current === data).
If no match is found, the method returns null.

remove(data) {
    let current = this.head;
    let previous = null;

    while (current) {
      if (current.data === data) {
        if (previous === null) {
          this.head = current.next;
        } else {
          previous.next = current.next;
        }
        this.size--;
        return current.data;
      }
      previous = current;
      current = current.next;
    }

    return null;
  }

Find the first occurrence of an element in the list

Declare variables current and index. These variables are used to keep track of the current node and its position in the linked list as the method iterates through the linked list to find the index of the node that matches the parameter.
Set current to the head of the linked list and the index to 0.
Loop through the linked list while current is not null.
1. If the data of current is equal to the data parameter passed into the function, the node whose index we are looking for has been found.
2. Return index, as it represents the position of the desired node.
As long as the current data node is not found, move to the next node in the linked list and increment the index by 1.
If no match is found, the method returns -1.

indexOf(data) {
    let current = this.head;
    let index = 0;

    while (current) {
      if (current.data === data) {
        return index;
      }

      current = current.next;
      index++;
    }

    return -1;
  }

Simple helper methods

// Check if list is empty
isEmpty() {
    return this.size === 0;
  }

// Return the number of elements
  size() {
    return this.size;
  }

// Return the head node
  getHead() {
    return this.head;
  }

// Return the tail node
  getTail() {
    return this.tail;
  }

// Remove all elements
  clear() {
    this.head = null;
    this.tail = null;
    this.size = 0;
  }

Conclusion

Linked lists are an important data structure used in computer science and programming. In this article, we covered how to create a Node class and a LinkedList class in JavaScript, and how to use them to add, insert, remove, and perform various other functions on a linked list. By understanding the basic principles of linked lists and the common functions used to manipulate them, you will be better equipped to use them in your own code.

If you're interested in learning more about linked lists and their advanced concepts and techniques, be sure to check out my upcoming blog posts. I'll be diving deeper into topics specific to linked lists, such as doubly linked lists and circular linked lists, and more. With this knowledge, you'll have a strong foundation for using linked lists in your own programming projects. Stay tuned for more!