JavaScript Data Structures - Doubly Linked List

Definition

A doubly linked list is a linear data structure that represents a collection of elements, where each element points both to the next and the previous one. The first element in the doubly linked list is the head and the last element is the tail.

Each element of a doubly linked list data structure must have the following properties:

• value: The value of the element
• next: A pointer to the next element in the linked list (null if there is none)
• previous: A pointer to the previous element in the linked list (null if there is none)

The main properties of a doubly linked list data structure are:

• size: The number of elements in the doubly linked list
• head: The first element in the doubly linked list
• tail: The last element in the doubly linked list

The main operations of a doubly linked list data structure are:

• insertAt: Inserts an element at the specific index
• removeAt: Removes the element at the specific index
• getAt: Retrieves the element at the specific index
• clear: Empties the doubly linked list
• reverse: Reverses the order of elements in the doubly linked list

Implementation

class DoublyLinkedList {
  constructor() {
    this.nodes = [];
  }

  get size() {
    return this.nodes.length;
  }

  get head() {
    return this.size ? this.nodes[0] : null;
  }

  get tail() {
    return this.size ? this.nodes[this.size - 1] : null;
  }

  insertAt(index, value) {
    const previousNode = this.nodes[index - 1] || null;
    const nextNode = this.nodes[index] || null;
    const node = { value, next: nextNode, previous: previousNode };

    if (previousNode) previousNode.next = node;
    if (nextNode) nextNode.previous = node;
    this.nodes.splice(index, 0, node);
  }

  insertFirst(value) {
    this.insertAt(0, value);
  }

  insertLast(value) {
    this.insertAt(this.size, value);
  }

  getAt(index) {
    return this.nodes[index];
  }

  removeAt(index) {
    const previousNode = this.nodes[index - 1] || null;
    const nextNode = this.nodes[index + 1] || null;

    if (previousNode) previousNode.next = nextNode;
    if (nextNode) nextNode.previous = previousNode;

    return this.nodes.splice(index, 1);
  }

  clear() {
    this.nodes = [];
  }

  reverse() {
    this.nodes = this.nodes.reduce((acc, { value }) => {
      const nextNode = acc[0] || null;
      const node = { value, next: nextNode, previous: null };
      if (nextNode) nextNode.previous = node;
      return [node, ...acc];
    }, []);
  }

  *[Symbol.iterator]() {
    yield* this.nodes;
  }
}

• Create a class with a constructor that initializes an empty array, nodes, for each instance.
• Define a size getter, that returns that uses Array.prototype.length to return the number of elements in the nodes array.
• Define a head getter, that returns the first element of the nodes array or null if empty.
• Define a tail getter, that returns the last element of the nodes array or null if empty.
• Define an insertAt() method, which uses Array.prototype.splice() to add a new object in the nodes array, updating the next and previous keys of the previous and next elements respectively.
• Define two convenience methods, insertFirst() and insertLast() that use the insertAt() method to insert a new element at the start or end of the nodes array respectively.
• Define a getAt() method, which retrieves the element in the given index.
• Define a removeAt() method, which uses Array.prototype.splice() to remove an object in the nodes array, updating the next and previous keys of the previous and next elements respectively.
• Define a clear() method, which empties the nodes array.
• Define a reverse() method, which uses Array.prototype.reduce() and the spread operator (...) to reverse the order of the nodes array, updating the next and previous keys of each element appropriately.
• Define a generator method for Symbol.iterator, which delegates to the nodes array's iterator using the yield* syntax.

const list = new DoublyLinkedList();

list.insertFirst(1);
list.insertFirst(2);
list.insertFirst(3);
list.insertLast(4);
list.insertAt(3, 5);

list.size;                      // 5
list.head.value;                // 3
list.head.next.value;           // 2
list.tail.value;                // 4
list.tail.previous.value;       // 5
[...list.map(e => e.value)];    // [3, 2, 1, 5, 4]

list.removeAt(1);               // 2
list.getAt(1).value;            // 1
list.head.next.value;           // 1
[...list.map(e => e.value)];    // [3, 1, 5, 4]

list.reverse();
[...list.map(e => e.value)];    // [4, 5, 1, 3]

list.clear();
list.size;                      // 0