Enqueue Using Linked List in DSA Typescript

Mental Model

Enqueue means adding a new item at the end of a line. Using a linked list, we keep track of the last item to add new items quickly.

Analogy: Imagine a line of people holding hands. To add a new person, you only need to hold the last person's hand and link the new person there.

head -> 1 -> 2 -> 3 -> null
                 ↑tail

Dry Run Walkthrough

Input: queue: 1 -> 2 -> 3 -> null, enqueue value 4

Goal: Add value 4 at the end of the queue using linked list enqueue operation

Step 1: Create new node with value 4

head -> 1 -> 2 -> 3 -> null
                 ↑tail
newNode: 4 -> null

Why: We need a new node to add to the queue

Step 2: Link current tail's next to new node

head -> 1 -> 2 -> 3 -> 4 -> null
                 ↑tail
newNode: 4 -> null

Why: Attach new node at the end of the queue

Step 3: Update tail pointer to new node

head -> 1 -> 2 -> 3 -> 4 -> null
                      ↑tail

Why: Tail must point to the last node for future enqueues

Result:

head -> 1 -> 2 -> 3 -> 4 -> null
                      ↑tail

Annotated Code

DSA Typescript

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

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

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

  enqueue(value: number): void {
    const newNode = new Node(value); // create new node
    if (!this.tail) { // if queue empty
      this.head = newNode; // head and tail are new node
      this.tail = newNode;
      return;
    }
    this.tail.next = newNode; // link new node at tail
    this.tail = newNode; // update tail pointer
  }

  printQueue(): void {
    let curr = this.head;
    let result = '';
    while (curr) {
      result += curr.value + ' -> ';
      curr = curr.next;
    }
    result += 'null';
    console.log(result);
  }
}

// Driver code
const queue = new Queue();
queue.enqueue(1);
queue.enqueue(2);
queue.enqueue(3);
queue.printQueue(); // before enqueue 4
queue.enqueue(4);
queue.printQueue(); // after enqueue 4

const newNode = new Node(value); // create new node

create new node to add

if (!this.tail) { // if queue empty

check if queue is empty to set head and tail

this.tail.next = newNode; // link new node at tail

attach new node after current tail

this.tail = newNode; // update tail pointer

move tail pointer to new last node

Output

1 -> 2 -> 3 -> null

1 -> 2 -> 3 -> 4 -> null

Complexity Analysis

Time: O(1) because we add the new node directly at the tail without traversing

Space: O(1) because we only create one new node for each enqueue

vs Alternative: Compared to array enqueue which may require shifting or resizing, linked list enqueue is faster and uses constant time

Edge Cases

Empty queue

Head and tail both point to the new node

DSA Typescript

if (!this.tail) { // if queue empty

Single element queue

New node is linked after tail and tail updates to new node

DSA Typescript

this.tail.next = newNode; // link new node at tail

When to Use This Pattern

When you need to add items to the end of a queue efficiently, use the enqueue linked list pattern with a tail pointer to avoid traversing the whole list.

Common Mistakes

Mistake: Not updating the tail pointer after adding the new node
Fix: Always set tail = newNode after linking to keep tail correct

Mistake: Not handling empty queue case where head and tail are null
Fix: Check if tail is null and set both head and tail to new node

Summary

Adds a new element at the end of the queue using a linked list.

Use when you want fast enqueue operations without shifting elements.

Keep a tail pointer to add new nodes in constant time.

Practice

1. What does the enqueue operation do in a linked list implementation of a queue?

easy

A. Removes the first element from the linked list

B. Adds a new element at the end (tail) of the linked list

C. Adds a new element at the beginning (head) of the linked list

D. Deletes the last element from the linked list

Solution

1. Step 1: Understand enqueue operation

   Enqueue means adding an element to the queue.

2. Step 2: Identify linked list position for enqueue

   In a linked list queue, new elements are added at the tail (end).

3. Final Answer:

   Adds a new element at the end (tail) of the linked list -> Option B

4. Quick Check:

   Enqueue = Add at tail [OK]

Hint: Enqueue always adds at the tail in linked list queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue operation
• Adding new element at head instead of tail
• Thinking enqueue removes elements
• Mixing linked list positions

2. Which of the following is the correct TypeScript syntax to create a new node for enqueue in a linked list?

easy

A. newNode = new Node();

B. let newNode = Node(value);

C. const newNode = new Node(value);

D. const newNode = Node.new(value);

Solution

1. Step 1: Recall TypeScript class instantiation

   To create an object, use new ClassName(params).

2. Step 2: Check options for correct syntax

   Only const newNode = new Node(value); correctly creates a new Node instance.

3. Final Answer:

   const newNode = new Node(value); -> Option C

4. Quick Check:

   Use 'new' keyword to instantiate class [OK]

Hint: Use 'new' keyword to create class instances in TypeScript [OK]

Common Mistakes:

• Omitting 'new' keyword when creating objects
• Using function call syntax without 'new'
• Incorrect property access like Node.new
• Not declaring variable with let or const

3. What will be the state of the queue after executing the following enqueue operations on an empty linked list queue?

enqueue(10); enqueue(20); enqueue(30);

medium

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

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

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

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

Solution

1. Step 1: Understand enqueue order

   Enqueue adds elements at the tail, so order is preserved as inserted.

2. Step 2: Trace each enqueue

   After enqueue(10): 10 -> null
   After enqueue(20): 10 -> 20 -> null
   After enqueue(30): 10 -> 20 -> 30 -> null

3. Final Answer:

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

4. Quick Check:

   Enqueue order = insertion order [OK]

Hint: Enqueue adds at tail, so order stays same as insertion [OK]

Common Mistakes:

• Reversing order of elements after enqueue
• Confusing enqueue with push at head
• Misreading linked list direction
• Ignoring null at the end

4. Consider this enqueue function snippet for a linked list queue:

enqueue(value: number) {
  const newNode = new Node(value);
  if (!this.tail) {
    this.head = newNode;
    this.tail = newNode;
    return;
  }
  this.tail.next = newNode;
  this.tail = newNode;
}

What is the bug in this code?

medium

A. It does not update the previous tail's next pointer to newNode

B. It incorrectly sets head to null when queue is empty

C. It creates newNode without value

D. It sets tail to null after enqueue

Solution

1. Step 1: Analyze enqueue logic

   When queue is empty, head and tail should point to newNode.

2. Step 2: Check linking of nodes

   Code sets tail = newNode but does not link old tail's next to newNode, breaking chain.

3. Final Answer:

   It does not update the previous tail's next pointer to newNode -> Option A

4. Quick Check:

   Link old tail's next to newNode [OK]

Hint: Always link old tail's next to new node when enqueueing [OK]

Common Mistakes:

• Forgetting to update old tail's next pointer
• Not handling empty queue case properly
• Setting tail to null accidentally
• Creating node without value parameter

5. You have a linked list queue with elements: 5 -> 15 -> 25 -> null.
After enqueue(35), what will be the correct linked list representation and tail node value?

hard

A. 5 -> 15 -> 25 -> 35 -> null, tail value = 25

B. 35 -> 5 -> 15 -> 25 -> null, tail value = 25

C. 5 -> 15 -> 25 -> null, tail value = 25

D. 5 -> 15 -> 25 -> 35 -> null, tail value = 35

Solution

1. Step 1: Understand enqueue effect on queue

   Enqueue adds new element at tail, so 35 goes after 25.

2. Step 2: Update tail pointer

   Tail must point to the new last node, which holds 35.

3. Final Answer:

   5 -> 15 -> 25 -> 35 -> null, tail value = 35 -> Option D

4. Quick Check:

   Enqueue adds at tail, tail updates to new node [OK]

Hint: Tail always points to last enqueued node [OK]

Common Mistakes:

• Adding new node at head instead of tail
• Not updating tail pointer after enqueue
• Confusing tail value with head value
• Leaving tail pointing to old last node