Insert at End of Doubly Linked List in DSA Typescript - Build from Scratch

Insert at End of Doubly Linked List

📖 Scenario: You are building a simple playlist manager. Each song is a node in a doubly linked list. You want to add new songs to the end of the playlist.

🎯 Goal: Create a doubly linked list and write code to insert a new node at the end. Then print the list from head to tail showing the song names.

📋 What You'll Learn

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

Create a DoublyLinkedList class with head and tail properties

Write a method insertAtEnd(data: string) to add a new node at the end

Write a method printList() to print the list from head to tail

💡 Why This Matters

🌍 Real World

Doubly linked lists are used in music players, browsers (history), and other apps where you need to move forward and backward through items.

💼 Career

Understanding linked lists helps in software engineering roles that involve data structure design, memory management, and building efficient algorithms.

1

Create Node and DoublyLinkedList classes

Create a Node class with data (string), prev (Node or null), and next (Node or null). Then create a DoublyLinkedList class with head and tail properties initialized to null.

DSA Typescript

// Your code here

Hint

Define Node with constructor setting data, prev, and next. Define DoublyLinkedList with head and tail set to null.

2

Add insertAtEnd method

Inside the DoublyLinkedList class, add a method insertAtEnd(data: string): void that creates a new Node with data and inserts it at the end of the list. Update head and tail accordingly.

DSA Typescript

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

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

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

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

    // Add insertAtEnd method here
}

Hint

Create a new node. If list is empty, set head and tail to new node. Otherwise, link new node after tail and update tail.

3

Add printList method

Inside the DoublyLinkedList class, add a method printList(): void that prints the list from head to tail. Print the data of each node separated by -> and end with null.

DSA Typescript

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

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

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

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

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

    // Add printList method here
}

Hint

Start from head, loop while node is not null, add data and arrow to a string, then print it.

4

Insert songs and print the playlist

Create a DoublyLinkedList instance called playlist. Insert these songs at the end in order: 'Song A', 'Song B', 'Song C'. Then call playlist.printList() to print the playlist.

DSA Typescript

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

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

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

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

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

    printList(): void {
        let current = this.head;
        let result = '';
        while (current !== null) {
            result += current.data + ' -> ';
            current = current.next;
        }
        result += 'null';
        console.log(result);
    }
}

// Your code here

Hint

Create playlist as new DoublyLinkedList(). Use insertAtEnd to add songs in order. Call printList() to show the list.

Practice

1. What is the main purpose of inserting a node at the end of a doubly linked list?

easy

A. To add a new element after the last node

B. To remove the first node

C. To reverse the list

D. To find the middle node

Solution

1. Step 1: Understand the structure of a doubly linked list

   A doubly linked list has nodes connected forward and backward, allowing traversal in both directions.

2. Step 2: Purpose of insertion at the end

   Inserting at the end means adding a new node after the current last node, updating pointers accordingly.

3. Final Answer:

   To add a new element after the last node -> Option A

4. Quick Check:

   Insert at end = add after last node [OK]

Hint: Insert at end means add after last node [OK]

Common Mistakes:

• Confusing insertion at end with deletion
• Thinking insertion reverses the list
• Assuming insertion happens at the start

2. Which of the following TypeScript code snippets correctly initializes a new node for a doubly linked list?

easy

A. const newNode = { data: 5, next: undefined };

B. const newNode = { data: 5, next: null, prev: null };

C. const newNode = { value: 5, nextNode: null };

D. const newNode = { data: 5 };

Solution

1. Step 1: Identify required properties for a doubly linked list node

   A node must have data, next pointer, and prev pointer to link both directions.

2. Step 2: Check each option for correct properties

   const newNode = { data: 5, next: null, prev: null }; has data, next, and prev all properly set. Others miss prev or use wrong property names.

3. Final Answer:

   const newNode = { data: 5, next: null, prev: null }; -> Option B

4. Quick Check:

   Node needs data, next, prev [OK]

Hint: Node needs data, next, and prev properties [OK]

Common Mistakes:

• Omitting prev pointer
• Using wrong property names like value or nextNode
• Setting next or prev as undefined instead of null

3. Given the following TypeScript code for inserting at the end of a doubly linked list, what will be the printed list after insertion?

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

class DoublyLinkedList {
  head: Node | null = null;

  insertAtEnd(data: number) {
    const newNode = new Node(data);
    if (!this.head) {
      this.head = newNode;
      return;
    }
    let temp = this.head;
    while (temp.next) {
      temp = temp.next;
    }
    temp.next = newNode;
    newNode.prev = temp;
  }

  printList() {
    let temp = this.head;
    let result = '';
    while (temp) {
      result += temp.data + ' -> ';
      temp = temp.next;
    }
    result += 'null';
    console.log(result);
  }
}

const dll = new DoublyLinkedList();
dll.insertAtEnd(10);
dll.insertAtEnd(20);
dll.insertAtEnd(30);
dll.printList();

medium

A. 10 -> 20 -> 30 -> null

B. 30 -> 20 -> 10 -> null

C. 10 -> 20 -> null

D. null

Solution

1. Step 1: Trace insertAtEnd calls

   First insert adds 10 as head. Second adds 20 after 10. Third adds 30 after 20.

2. Step 2: Trace printList output

   Print starts at head (10), then 20, then 30, then null.

3. Final Answer:

   10 -> 20 -> 30 -> null -> Option A

4. Quick Check:

   Inserted nodes in order = 10 -> 20 -> 30 [OK]

Hint: Insertion at end appends nodes in order [OK]

Common Mistakes:

• Confusing order and printing reversed list
• Missing last inserted node
• Printing null instead of list

4. Identify the error in the following TypeScript method for inserting at the end of a doubly linked list:

insertAtEnd(data: number) {
  const newNode = new Node(data);
  if (!this.head) {
    this.head = newNode;
  }
  let temp = this.head;
  while (temp.next) {
    temp = temp.next;
  }
  temp.next = newNode;
  newNode.prev = temp;
}

medium

A. newNode should be created outside the method

B. Incorrect assignment of newNode.prev

C. Loop condition should be temp !== null

D. Missing return after setting head when list is empty

Solution

1. Step 1: Analyze empty list case

   If head is null, method sets head to newNode but continues to loop and assign next, causing error.

2. Step 2: Fix by adding return after setting head

   Adding return after setting head prevents further code execution and avoids null pointer errors.

3. Final Answer:

   Missing return after setting head when list is empty -> Option D

4. Quick Check:

   Return after head set avoids errors [OK]

Hint: Return immediately after setting head if list empty [OK]

Common Mistakes:

• Not returning after setting head
• Misunderstanding loop condition
• Incorrect pointer assignments

5. You want to insert a new node with data 40 at the end of a doubly linked list that currently has nodes 10 <-> 20 <-> 30. After insertion, what will be the correct state of the list including the prev and next pointers of the new node?

hard

A. 10 <-> 20 <-> 30, with 40.prev and 40.next both null (not linked)

B. 40 <-> 10 <-> 20 <-> 30, where 40.next points to 10 and 40.prev is null

C. 10 <-> 20 <-> 30 <-> 40, where 40.prev points to 30 and 40.next is null

D. 10 <-> 20 <-> 30 <-> 40, where 40.prev and 40.next both point to null

Solution

1. Step 1: Understand insertion at end

   The new node 40 should be linked after 30, so 30.next = 40 and 40.prev = 30.

2. Step 2: Confirm pointers of new node

   Since 40 is last, 40.next must be null to mark list end.

3. Final Answer:

   10 <-> 20 <-> 30 <-> 40, where 40.prev points to 30 and 40.next is null -> Option C

4. Quick Check:

   New tail prev = old tail, next = null [OK]

Hint: New tail node prev = old tail, next = null [OK]

Common Mistakes:

• Setting new node's next pointer incorrectly
• Not linking new node's prev pointer
• Inserting new node at start instead of end