Why Insert at Beginning Head Insert in DSA Typescript?

The Big Idea

What if you could add new things to the front of your list instantly, without rewriting everything?

The Scenario

Imagine you have a list of your favorite songs written on paper. Every time you discover a new favorite, you want to add it to the very start of your list so you see it first. Doing this by rewriting the entire list every time is tiring and slow.

The Problem

Manually rewriting the whole list to add a new item at the start takes a lot of time and can cause mistakes like skipping songs or mixing the order. It's hard to keep the list neat and updated quickly.

The Solution

Using the "Insert at Beginning" method in a linked list lets you add a new item right at the start instantly. You just create a new node and point it to the current first node, then update the head to this new node. No rewriting needed!

Before vs After

✗ Before

let list = ["Song1", "Song2", "Song3"];
// To add "NewSong" at start
list = ["NewSong", ...list];

✓ After

class Node {
  value: string;
  next: Node | null = null;
  constructor(value: string) { this.value = value; }
}

class LinkedList {
  head: Node | null = null;
  insertAtBeginning(value: string) {
    const newNode = new Node(value);
    newNode.next = this.head;
    this.head = newNode;
  }
}

What It Enables

This lets you quickly add new important items to the front of your list without disturbing the rest, making updates fast and error-free.

Real Life Example

Think of a to-do list app where the most urgent tasks appear at the top immediately when you add them, so you never miss what's important.

Key Takeaways

Manually adding at the start is slow and error-prone.

Head insert adds new nodes instantly at the front.

It keeps your list updated efficiently and cleanly.

Practice

1. What does the "Insert at Beginning" operation do in a singly linked list?

easy

A. Adds a new node at the start of the list

B. Adds a new node at the end of the list

C. Deletes the first node of the list

D. Deletes the last node of the list

Solution

1. Step 1: Understand the operation

   Inserting at the beginning means adding a new node before the current first node.

2. Step 2: Effect on the list

   The new node becomes the head, pushing the old head to second position.

3. Final Answer:

   Adds a new node at the start of the list -> Option A

4. Quick Check:

   Insert at beginning = Add new head [OK]

Hint: Insert at beginning means new head node [OK]

Common Mistakes:

• Confusing insert at beginning with insert at end
• Thinking it deletes nodes
• Assuming it changes the tail node

2. Which of the following is the correct TypeScript syntax to insert a new node at the beginning of a singly linked list?

easy

A. newNode = head; head.next = newNode;

B. head = newNode.next; newNode = head;

C. head.next = newNode; newNode = head;

D. newNode.next = head; head = newNode;

Solution

1. Step 1: Link new node to current head

   Set newNode.next to point to current head node.

2. Step 2: Update head to new node

   Assign head to newNode so it becomes the first node.

3. Final Answer:

   newNode.next = head; head = newNode; -> Option D

4. Quick Check:

   Link new node to head, then update head [OK]

Hint: Set newNode.next to head, then head = newNode [OK]

Common Mistakes:

• Assigning head before linking newNode.next
• Swapping assignments order
• Using head.next instead of newNode.next

3. What will be the linked list after executing the following TypeScript code?

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

let head: Node | null = new Node(10);
let newNode = new Node(5);
newNode.next = head;
head = newNode;

medium

A. 10 -> null

B. 10 -> 5 -> null

C. 5 -> 10 -> null

D. 5 -> null

Solution

1. Step 1: Initial list setup

   head points to node with value 10, next is null.

2. Step 2: Insert newNode at beginning

   newNode (5) points to head (10), then head updated to newNode.

3. Final Answer:

   5 -> 10 -> null -> Option C

4. Quick Check:

   New node 5 becomes head, points to 10 [OK]

Hint: New node points to old head, then becomes new head [OK]

Common Mistakes:

• Confusing order of nodes after insertion
• Forgetting to update head
• Assuming newNode.next is null

4. Identify the error in this TypeScript code that tries to insert a node at the beginning of a linked list:

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

let head: Node | null = null;
let newNode = new Node(7);
head.next = newNode;
head = newNode;

medium

A. head is null, so head.next causes an error

B. newNode is not linked to head

C. head should be assigned before newNode.next

D. Missing constructor call for newNode

Solution

1. Step 1: Check initial head value

   head is null, so accessing head.next is invalid and causes runtime error.

2. Step 2: Correct insertion approach

   Assign newNode.next = head (which is null), then head = newNode.

3. Final Answer:

   head is null, so head.next causes an error -> Option A

4. Quick Check:

   Cannot access property on null head [OK]

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

Common Mistakes:

• Trying to access next on null head
• Not linking newNode.next to head
• Assigning head after using head.next

5. Given a singly linked list with nodes 20 -> 30 -> 40 -> null, what will be the list after inserting 10 at the beginning using head insert method?

hard

A. 20 -> 30 -> 40 -> 10 -> null

B. 10 -> 20 -> 30 -> 40 -> null

C. 30 -> 40 -> 10 -> 20 -> null

D. 10 -> 30 -> 40 -> 20 -> null

Solution

1. Step 1: Understand initial list

   List is 20 -> 30 -> 40 -> null, head points to 20.

2. Step 2: Insert 10 at beginning

   Create new node 10, set newNode.next = head (20), then head = newNode.

3. Final Answer:

   10 -> 20 -> 30 -> 40 -> null -> Option B

4. Quick Check:

   New node 10 becomes head, old list follows [OK]

Hint: New node points to old head, becomes new head [OK]

Common Mistakes:

• Appending new node at end instead of beginning
• Mixing node order after insertion
• Not updating head pointer