Doubly Linked List Structure and Node Design in DSA Typescript - Build from Scratch

Doubly Linked List Structure and Node Design

📖 Scenario: Imagine you are building a simple music playlist app. Each song in the playlist can move forward or backward easily. To do this, you will create a doubly linked list where each song is a node connected to the previous and next songs.

🎯 Goal: You will build the basic structure of a doubly linked list by creating a Node class and a DoublyLinkedList class with methods to add songs. This will help you understand how nodes connect in both directions.

📋 What You'll Learn

Create a Node class with data, prev, and next properties

Create a DoublyLinkedList class with head and tail properties

Add a method append to add new nodes to the end of the list

Print the list from head to tail showing song names separated by ' <-> '

💡 Why This Matters

🌍 Real World

Doubly linked lists are used in music players, web browsers (back and forward navigation), and undo-redo features where moving in both directions is needed.

💼 Career

Understanding doubly linked lists helps in software development roles that require efficient data manipulation and navigation, such as building user interfaces, games, or complex data structures.

1

Create the Node class

Create a Node class with a constructor that takes a data string parameter. Add public properties data (string), prev (Node or null), and next (Node or null). Initialize prev and next to null.

DSA Typescript

// Your code here

Hint

Think of each Node as a song in the playlist. It needs to know its name (data) and links to the previous and next songs (prev and next).

2

Create the DoublyLinkedList class with head and tail

Create a DoublyLinkedList class with public properties head and tail, both initialized to null. These will track the first and last nodes in the list.

DSA Typescript

class Node {
    public data: string;
    public prev: Node | null;
    public next: Node | null;

    constructor(data: string) {
        this.data = data;
        this.prev = null;
        this.next = null;
    }
}

// Your code here

Hint

The DoublyLinkedList keeps track of the first song (head) and the last song (tail) in the playlist.

3

Add an append method to add nodes to the list

Inside the DoublyLinkedList class, add a public method append that takes a data string. Create a new Node with this data. If the list is empty (head is null), set both head and tail to the new node. Otherwise, link the new node after tail and update tail to the new node.

DSA Typescript

class Node {
    public data: string;
    public prev: Node | null;
    public next: Node | null;

    constructor(data: string) {
        this.data = data;
        this.prev = null;
        this.next = null;
    }
}

class DoublyLinkedList {
    public head: Node | null;
    public tail: Node | null;

    constructor() {
        this.head = null;
        this.tail = null;
    }

    // Your code here
}

Hint

When adding a new song, if the playlist is empty, the new song is both the first and last. Otherwise, connect it after the last song and update the tail.

4

Print the list from head to tail

Add code to create a DoublyLinkedList instance called playlist. Append these songs in order: 'Song A', 'Song B', 'Song C'. Then, print the playlist from head to tail by traversing nodes and joining their data with ' <-> '. Use console.log to display the result.

DSA Typescript

class Node {
    public data: string;
    public prev: Node | null;
    public next: Node | null;

    constructor(data: string) {
        this.data = data;
        this.prev = null;
        this.next = null;
    }
}

class DoublyLinkedList {
    public head: Node | null;
    public tail: Node | null;

    constructor() {
        this.head = null;
        this.tail = null;
    }

    public append(data: string): void {
        const newNode = new Node(data);
        if (this.head === null) {
            this.head = newNode;
            this.tail = newNode;
        } else {
            if (this.tail) {
                this.tail.next = newNode;
                newNode.prev = this.tail;
                this.tail = newNode;
            }
        }
    }
}

// Your code here

Hint

Start from the head and move to the next node until you reach the end (null). Collect each song's name and join them with ' <-> '.

Practice

1. What is the main difference between a doubly linked list node and a singly linked list node?

easy

A. A doubly linked list node has pointers to both the previous and next nodes.

B. A doubly linked list node stores data in a different format.

C. A doubly linked list node can only point to the next node.

D. A doubly linked list node does not store any data.

Solution

1. Step 1: Understand node pointers in singly linked list

   A singly linked list node has only one pointer to the next node.

2. Step 2: Understand node pointers in doubly linked list

   A doubly linked list node has two pointers: one to the previous node and one to the next node.

3. Final Answer:

   A doubly linked list node has pointers to both the previous and next nodes. -> Option A

4. Quick Check:

   Doubly linked list nodes have two pointers [OK]

Hint: Remember: doubly linked means two directions [OK]

Common Mistakes:

• Thinking doubly linked nodes only point forward
• Confusing data storage with pointer structure
• Assuming doubly linked nodes do not store data

2. Which of the following is the correct TypeScript syntax to define a doubly linked list node class with data of type number?

easy

A. class Node { data: number; next: Node | null; prev: Node | null; constructor(data: number) { this.data = data; this.next = null; this.prev = null; } }

B. class Node { data: number; next: Node; prev: Node; constructor(data: number) { this.data = data; this.next = null; this.prev = null; } }

C. class Node { data: number; next: Node; prev: Node; constructor(data: number) { this.data = data; } }

D. class Node { data: number; next: null; prev: null; constructor(data: number) { this.data = data; this.next = null; this.prev = null; } }

Solution

1. Step 1: Check property types for next and prev

   Since next and prev can be null initially, their types must allow null (Node | null).

2. Step 2: Verify constructor initializes all properties

   The constructor sets data and initializes next and prev to null correctly.

3. Final Answer:

   class Node { data: number; next: Node | null; prev: Node | null; constructor(data: number) { this.data = data; this.next = null; this.prev = null; } } -> Option A

4. Quick Check:

   Allow null for next and prev in TypeScript [OK]

Hint: Use 'Node | null' for nullable pointers [OK]

Common Mistakes:

• Not allowing next and prev to be null
• Forgetting to initialize next and prev in constructor
• Setting next and prev types incorrectly

3. What will be the output of the following TypeScript code snippet?

class Node {
  data: number;
  next: Node | null;
  prev: Node | null;
  constructor(data: number) {
    this.data = data;
    this.next = null;
    this.prev = null;
  }
}

const node1 = new Node(10);
const node2 = new Node(20);
node1.next = node2;
node2.prev = node1;

console.log(node1.next?.data);
console.log(node2.prev?.data);

medium

A. Error at runtime

B. 10\n20

C. undefined\nundefined

D. 20\n10

Solution

1. Step 1: Trace node1.next and node2.prev assignments

   node1.next points to node2, so node1.next.data is 20. node2.prev points to node1, so node2.prev.data is 10.

2. Step 2: Check console.log outputs

   console.log(node1.next?.data) prints 20, console.log(node2.prev?.data) prints 10.

3. Final Answer:

   20 10 -> Option D

4. Quick Check:

   next and prev pointers give 20 and 10 respectively [OK]

Hint: Follow next and prev pointers to get data [OK]

Common Mistakes:

• Confusing which node data is printed
• Ignoring optional chaining and assuming errors
• Mixing up prev and next pointers

4. Identify the error in the following TypeScript code for a doubly linked list node and select the fix.

class Node {
  data: number;
  next: Node;
  prev: Node;
  constructor(data: number) {
    this.data = data;
    this.next = null;
    this.prev = null;
  }
}

medium

A. Change data type from number to any.

B. Remove the constructor to fix the error.

C. Change next and prev types to 'Node | null' to allow null assignment.

D. Initialize next and prev with new Node() instead of null.

Solution

1. Step 1: Analyze type declarations and assignments

   next and prev are typed as Node but assigned null, which is invalid in TypeScript.

2. Step 2: Fix type to allow null

   Changing next and prev types to 'Node | null' allows null assignment and fixes the error.

3. Final Answer:

   Change next and prev types to 'Node | null' to allow null assignment. -> Option C

4. Quick Check:

   Nullable pointers need 'Node | null' type [OK]

Hint: Allow null in pointer types to avoid errors [OK]

Common Mistakes:

• Leaving next and prev as non-nullable types
• Trying to assign new Node() without data
• Removing constructor unnecessarily

5. You want to insert a new node with data 30 between two existing nodes in a doubly linked list: nodeA and nodeB, where nodeA.next = nodeB and nodeB.prev = nodeA. Which sequence of pointer updates correctly inserts the new node?

hard

A. nodeA.next = newNode; nodeB.prev = newNode; newNode.next = nodeB; newNode.prev = nodeA;

B. newNode.prev = nodeA; newNode.next = nodeB; nodeA.next = newNode; nodeB.prev = newNode;

C. newNode.next = nodeA; newNode.prev = nodeB; nodeA.prev = newNode; nodeB.next = newNode;

D. nodeA.prev = newNode; nodeB.next = newNode; newNode.prev = nodeB; newNode.next = nodeA;

Solution

1. Step 1: Set newNode pointers to link between nodeA and nodeB

   newNode.prev must point to nodeA and newNode.next must point to nodeB to fit between them.

2. Step 2: Update nodeA and nodeB pointers to include newNode

   nodeA.next should point to newNode and nodeB.prev should point to newNode to complete the insertion.

3. Final Answer:

   newNode.prev = nodeA; newNode.next = nodeB; nodeA.next = newNode; nodeB.prev = newNode; -> Option B

4. Quick Check:

   Link newNode between nodeA and nodeB correctly [OK]

Hint: Link newNode pointers first, then update neighbors [OK]

Common Mistakes:

• Mixing up prev and next pointers
• Updating neighbors before setting newNode pointers
• Reversing nodeA and nodeB in assignments