Delete from Beginning of Doubly Linked List in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output after deleting the first node from a doubly linked list

What is the printed state of the doubly linked list after deleting the first node?

DSA Typescript

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

class DoublyLinkedList {
  head: Node | null;
  constructor() {
    this.head = null;
  }

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

  deleteFromBeginning() {
    if (!this.head) return;
    if (!this.head.next) {
      this.head = null;
      return;
    }
    this.head = this.head.next;
    this.head.prev = null;
  }

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

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

A 20 <-> 30 <-> null

B 10 <-> 20 <-> 30 <-> null

C 30 <-> null

D null

💡 Hint

Deleting from beginning removes the first node and updates head.

Initially, the list is 10 <-> 20 <-> 30 <-> null. After deleting the first node (10), the head moves to 20, so the list becomes 20 <-> 30 <-> null.

🧠 Conceptual

intermediate

Effect of deleting from an empty doubly linked list

What happens when you try to delete the first node from an empty doubly linked list?

A Creates a new empty node as head.

B Throws a runtime error because head is null.

C Nothing happens; the list remains empty without error.

D Deletes a non-existent node and corrupts the list.

💡 Hint

Check the condition when head is null before deletion.

The deleteFromBeginning method checks if the list is empty (head is null) and returns immediately, so no error or change occurs.

❓ Predict Output

advanced

Output after deleting from a single-node doubly linked list

What is the printed state of the doubly linked list after deleting the first node when the list has only one node?

DSA Typescript

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

class DoublyLinkedList {
  head: Node | null;
  constructor() {
    this.head = null;
  }

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

  deleteFromBeginning() {
    if (!this.head) return;
    if (!this.head.next) {
      this.head = null;
      return;
    }
    this.head = this.head.next;
    this.head.prev = null;
  }

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

const dll = new DoublyLinkedList();
dll.append(100);
dll.deleteFromBeginning();
dll.printList();

A null

B 100 <-> null

C Error: Cannot read property 'prev' of null

D 100

💡 Hint

Deleting the only node should leave the list empty.

Since the list has only one node, deleting it sets head to null, so printing shows 'null'.

🔧 Debug

advanced

Identify the bug in this deleteFromBeginning method

What error will this code produce when deleting the first node from a doubly linked list with multiple nodes?

DSA Typescript

deleteFromBeginning() {
  if (!this.head) return;
  this.head = this.head.next;
  this.head.prev = null;
}

A ReferenceError: head is not defined

B No error; works correctly for all cases

C SyntaxError due to missing return statement

D TypeError: Cannot set property 'prev' of null when list has one node

💡 Hint

Consider what happens if the list has only one node.

If the list has only one node, this.head.next is null, so this.head becomes null. Then trying to set this.head.prev causes a TypeError.

🚀 Application

expert

Number of nodes after multiple deletions from beginning

Given a doubly linked list with nodes 5 <-> 15 <-> 25 <-> 35 <-> 45 <-> null, after deleting from the beginning three times, how many nodes remain?

A 3

B 2

C 4

D 1

💡 Hint

Each deletion removes one node from the start.

Starting with 5 nodes, deleting 3 nodes from the beginning leaves 2 nodes remaining.

Practice

1. What happens to the head pointer when you delete the first node from a doubly linked list?

easy

A. It moves to the previous node

B. It becomes null immediately

C. It stays pointing to the deleted node

D. It moves to the next node in the list

Solution

1. Step 1: Understand the role of head pointer

   The head pointer always points to the first node in the list.

2. Step 2: Effect of deleting the first node

   When the first node is deleted, head must point to the next node to keep the list accessible.

3. Final Answer:

   It moves to the next node in the list -> Option D

4. Quick Check:

   head points to next node after deletion [OK]

Hint: Head always points to the first valid node [OK]

Common Mistakes:

• Setting head to null without checking if list is empty
• Not updating head after deletion
• Pointing head to deleted node

2. Which of the following is the correct TypeScript syntax to delete the first node from a doubly linked list assuming head is the pointer to the first node?

easy

A. head = head.next; head.prev = null;

B. head.prev = null; head = head.next;

C. head.next = null; head = head.prev;

D. head = null; head.next = null;

Solution

1. Step 1: Move head to next node

   Assign head to head.next to remove first node reference.

2. Step 2: Set new head's prev to null

   Set head.prev to null to disconnect backward link from old first node.

3. Final Answer:

   head = head.next; head.prev = null; -> Option A

4. Quick Check:

   Correct syntax to update head and prev pointer [OK]

Hint: Update head first, then fix prev pointer [OK]

Common Mistakes:

• Setting head.prev before moving head
• Assigning head.prev to null before head changes
• Setting head.next to null incorrectly

3. Given the doubly linked list: 10 <-> 20 <-> 30 <-> null, what will be the list after deleting the first node?

medium

A. 20 <-> 30 <-> null

B. 10 <-> 30 <-> null

C. 30 <-> null

D. null

Solution

1. Step 1: Identify the first node

   The first node is 10, which will be removed.

2. Step 2: Update head to next node

   Head moves to 20, and 20.prev is set to null.

3. Final Answer:

   20 <-> 30 <-> null -> Option A

4. Quick Check:

   First node removed, list starts at 20 [OK]

Hint: After deletion, list starts from second node [OK]

Common Mistakes:

• Not updating prev pointer of new head
• Removing wrong node
• Leaving old head in list

4. Consider this TypeScript code snippet to delete the first node from a doubly linked list:

if (head !== null) {
  head = head.next;
  head.prev = null;
}

What error can occur here?

medium

A. Logical error: head.prev should not be set to null

B. Syntax error due to missing semicolon

C. Runtime error if list has only one node

D. No error, code is correct

Solution

1. Step 1: Check condition when list has one node

   If list has one node, head.next is null after deletion.

2. Step 2: Accessing head.prev causes error

   head becomes null, so head.prev causes runtime error (cannot access property of null).

3. Final Answer:

   Runtime error if list has only one node -> Option C

4. Quick Check:

   Null pointer access error possible [OK]

Hint: Check if head is null before accessing head.prev [OK]

Common Mistakes:

• Not checking if head is null after moving to next
• Assuming list always has multiple nodes
• Ignoring runtime null errors

5. You have a doubly linked list with nodes: 5 <-> 15 <-> 25 <-> 35 <-> null. Write the correct sequence of operations to delete the first node safely, considering the list might have only one node.

hard

A. head.prev = null; head = head.next;

B. if (head !== null) { head = head.next; if (head !== null) head.prev = null; }

C. if (head.next !== null) { head = head.next; head.prev = null; }

D. head = head.next; head.prev = null;

Solution

1. Step 1: Check if list is empty

   First, verify head is not null to avoid errors on empty list.

2. Step 2: Move head to next node and check again

   After moving head, check if new head is not null before setting prev to null.

3. Final Answer:

   if (head !== null) { head = head.next; if (head !== null) head.prev = null; } -> Option B

4. Quick Check:

   Safe deletion with null checks [OK]

Hint: Always check for null before accessing pointers [OK]

Common Mistakes:

• Not checking if head is null after moving
• Setting prev pointer without null check
• Assuming list has multiple nodes always