Queue Implementation Using Linked List in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a queue in data structures?

A queue is a collection where elements are added at the back and removed from the front, following the First-In-First-Out (FIFO) order.

beginner

Why use a linked list to implement a queue?

Using a linked list allows dynamic memory use and efficient enqueue and dequeue operations without shifting elements, unlike arrays.

beginner

In a linked list queue, what do 'front' and 'rear' pointers represent?

'Front' points to the first element to be dequeued, and 'rear' points to the last element where new elements are enqueued.

beginner

What happens when you dequeue from an empty queue implemented with a linked list?

Since the queue is empty, there is no element to remove; typically, the operation returns null or indicates underflow.

intermediate

How does enqueue operation work in a linked list queue?

Create a new node, link it at the rear's next, update rear to this new node. If queue was empty, front also points to the new node.

What order does a queue follow?

A First-In-First-Out (FIFO)

B Last-In-First-Out (LIFO)

C Random order

D Sorted order

A queue always removes elements in the order they were added, which is FIFO.

In a linked list queue, which pointer moves when you enqueue a new element?

A Front pointer

B Rear pointer

C Both front and rear pointers

D Neither pointer

The rear pointer moves to point to the newly added node at the end.

What is the time complexity of enqueue and dequeue operations in a linked list queue?

A O(1) for both

B O(n) for enqueue, O(1) for dequeue

C O(1) for enqueue, O(n) for dequeue

D O(n) for both

Both enqueue and dequeue operations take constant time O(1) in a linked list queue.

What happens to the front pointer after a dequeue operation?

A It becomes null always

B It stays the same

C It moves to the next node

D It moves to the rear

After removing the front node, the front pointer moves to the next node in the queue.

If the queue is empty, what should the rear pointer be?

A Point to the front node

B Point to itself

C Point to a dummy node

D Be null

When the queue is empty, both front and rear pointers should be null.

Explain how enqueue and dequeue operations work in a queue implemented using a linked list.

Think about how nodes link and how pointers move.

Describe the advantages of using a linked list over an array for implementing a queue.

Consider what happens when the queue grows or shrinks.

Practice

1. What is the main advantage of implementing a Queue using a linked list instead of an array?

easy

A. Simpler syntax for insertion and deletion

B. Faster random access to elements

C. Uses less memory than arrays

D. Dynamic size without fixed capacity limits

Solution

1. Step 1: Understand queue size limitations in arrays

   Arrays have fixed size or require resizing, which can be costly.

2. Step 2: Linked list dynamic size advantage

   Linked lists grow and shrink dynamically without fixed capacity limits.

3. Final Answer:

   Dynamic size without fixed capacity limits -> Option D

4. Quick Check:

   Linked list queues grow dynamically [OK]

Hint: Linked lists grow dynamically, arrays have fixed size [OK]

Common Mistakes:

• Thinking linked lists allow faster random access
• Assuming linked lists always use less memory
• Confusing syntax simplicity with performance

2. Which of the following is the correct way to add a new node to the end of a linked list queue in JavaScript?

easy

A. tail.next = newNode; tail = newNode;

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

C. newNode.next = tail; tail = newNode;

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

Solution

1. Step 1: Identify tail pointer usage in queue enqueue

   New nodes are added at the tail; tail.next points to new node.

2. Step 2: Update tail to new node

   After adding, tail must point to the new node to keep queue end updated.

3. Final Answer:

   tail.next = newNode; tail = newNode; -> Option A

4. Quick Check:

   Enqueue adds at tail [OK]

Hint: Enqueue adds at tail: tail.next = newNode; tail = newNode [OK]

Common Mistakes:

• Adding new node at head instead of tail
• Not updating tail pointer after insertion
• Confusing head and tail pointers

3. You want to implement a method peek() for a linked list queue that returns the front element without removing it. Which is the correct implementation?

easy

A. return this.head ? this.head.value : null;

B. return this.tail ? this.tail.value : null;

C. return this.head.next ? this.head.next.value : null;

D. return this.tail.next ? this.tail.next.value : null;

Solution

1. Step 1: Understand peek operation in queue

   Peek returns front element without removing it, which is at head in linked list queue.

2. Step 2: Check correct pointer for front element

   Head points to front node; tail points to end node. So return head.value if head exists.

3. Final Answer:

   return this.head ? this.head.value : null; -> Option A

4. Quick Check:

   Peek returns head value [OK]

Hint: Peek returns head value without dequeue [OK]

Common Mistakes:

• Returning tail value instead of head
• Accessing next of tail which is always null
• Returning head.next value which skips front

4. Consider this JavaScript code for a linked list queue:

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

class Queue {
  constructor() {
    this.head = null;
    this.tail = null;
  }
  enqueue(value) {
    const newNode = new Node(value);
    if (!this.tail) {
      this.head = newNode;
      this.tail = newNode;
      return;
    }
    this.tail.next = newNode;
    this.tail = newNode;
  }
  dequeue() {
    if (!this.head) return null;
    const val = this.head.value;
    this.head = this.head.next;
    if (!this.head) this.tail = null;
    return val;
  }
}

const q = new Queue();
q.enqueue(10);
q.enqueue(20);
q.dequeue();
q.enqueue(30);
q.dequeue();
q.enqueue(40);
q.enqueue(50);
q.dequeue();
console.log(q.head.value);

What will be printed?

medium

A. 20

B. 40

C. 50

D. 30

Solution

1. Step 1: Trace enqueue and dequeue operations

   Queue after operations: - enqueue 10: head=10, tail=10 - enqueue 20: head=10->20, tail=20 - dequeue: removes 10, head=20 - enqueue 30: head=20->30, tail=30 - dequeue: removes 20, head=30 - enqueue 40: head=30->40, tail=40 - enqueue 50: head=30->40->50, tail=50 - dequeue: removes 30, head=40

2. Step 2: Identify current head value

   After last dequeue, head points to node with value 40.

3. Final Answer:

   40 -> Option B

4. Quick Check:

   Final head value after operations = 40 [OK]

Hint: Track head after each dequeue to find current front [OK]

Common Mistakes:

• Forgetting to update tail when queue becomes empty
• Mixing up enqueue and dequeue order
• Assuming head value is unchanged after dequeue

5. The following dequeue method for a linked list queue has a bug:

dequeue() {
  if (!this.head) return null;
  const val = this.head.value;
  this.head = this.head.next;
  return val;
}

What is the problem?

medium

A. It removes the tail instead of the head

B. It returns the wrong value

C. It does not update tail when queue becomes empty

D. It causes infinite loop

Solution

1. Step 1: Analyze dequeue method behavior

   It removes head by moving head pointer to next node and returns old head value.

2. Step 2: Identify missing tail update

   If after removal head becomes null (queue empty), tail still points to old node causing inconsistency.

3. Final Answer:

   It does not update tail when queue becomes empty -> Option C

4. Quick Check:

   Empty queue must reset tail = null [OK]

Hint: Reset tail to null when head becomes null after dequeue [OK]

Common Mistakes:

• Ignoring tail pointer update on empty queue
• Confusing head and tail roles
• Assuming dequeue removes tail node