Enqueue Using Linked List in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new node with the given value.

DSA Typescript

class Node {
  value: number;
  next: Node | null;
  constructor(value: number) {
    this.value = [1];
    this.next = null;
  }
}

Drag options to blanks, or click blank then click option'

Anull

Bnext

Cvalue

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning 'null' instead of the passed value.

Assigning 'next' property instead of 'value'.

The constructor should assign the passed value to the node's value property.

2fill in blank

medium

Complete the code to check if the queue is empty.

DSA Typescript

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

  isEmpty(): boolean {
    return this.head [1] null;
  }
}

Drag options to blanks, or click blank then click option'

A===

B!==

C>=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using !== instead of ===, which reverses the logic.

Using > or < operators which are not suitable for null checks.

The queue is empty if head is exactly null.

3fill in blank

hard

Fix the error in the enqueue method to correctly add a new node at the tail.

DSA Typescript

enqueue(value: number): void {
  const newNode = new Node(value);
  if (this.tail) {
    this.tail.next = [1];
  }
  this.tail = newNode;
  if (!this.head) {
    this.head = newNode;
  }
}

Drag options to blanks, or click blank then click option'

AnewNode

Bthis.head

Cnull

Dthis.tail

Attempts:

3 left

💡 Hint

Common Mistakes

Setting 'this.tail.next' to 'this.head' or 'this.tail' causing loops.

Setting 'this.tail.next' to null which breaks the chain.

The current tail's next should point to the new node being added.

4fill in blank

hard

Fill both blanks to complete the dequeue method that removes the head node and returns its value.

DSA Typescript

dequeue(): number | null {
  if (this.head === null) {
    return null;
  }
  const value = this.head.value;
  this.head = this.head.[1];
  if (this.head === null) {
    this.[2] = null;
  }
  return value;
}

Drag options to blanks, or click blank then click option'

Anext

Btail

Chead

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Setting head to head.prev which does not exist in singly linked list.

Not updating tail when queue becomes empty.

To remove the head, set head to head.next. If head becomes null, tail must also be null.

5fill in blank

hard

Fill all three blanks to complete the printQueue method that prints all values in the queue.

DSA Typescript

printQueue(): void {
  let current = this.[1];
  while (current [2] null) {
    console.log(current.[3]);
    current = current.next;
  }
}

Drag options to blanks, or click blank then click option'

Ahead

B!==

Cvalue

Dtail

Attempts:

3 left

💡 Hint

Common Mistakes

Starting from tail instead of head.

Using '==' instead of '!==' in the loop condition.

Printing 'current.next' instead of 'current.value'.

Start from head, loop while current is not null, print current.value.

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