Enqueue Using Linked List in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'enqueue' mean in the context of a queue data structure?

Enqueue means adding an element to the end (rear) of the queue.

beginner

Why is a linked list a good choice to implement a queue?

A linked list allows efficient addition at the end and removal from the front without shifting elements, making enqueue and dequeue operations fast.

intermediate

In enqueue operation using a linked list, which pointer is updated to add the new node?

The 'rear' pointer is updated to point to the new node added at the end of the linked list.

beginner

What happens if you enqueue an element when the queue is empty?

Both 'front' and 'rear' pointers are set to the new node because it is the only element in the queue.

beginner

Show the state of the queue after enqueueing 10, 20, and 30 using a linked list.

Queue state: 10 -> 20 -> 30 -> null, where 'front' points to 10 and 'rear' points to 30.

What pointer do you update when enqueueing a new node in a linked list queue?

A tail.next

B front

C head

D rear

The 'rear' pointer is updated to point to the newly added node at the end.

What should happen to 'front' and 'rear' when enqueueing the first element in an empty queue?

A Only 'rear' is updated

B Only 'front' is updated

C Both 'front' and 'rear' point to the new node

D Neither pointer changes

When the queue is empty, both 'front' and 'rear' must point to the new node.

Which operation adds an element to the end of a queue?

A Enqueue

B Dequeue

C Peek

D Pop

Enqueue adds an element to the rear (end) of the queue.

What is the time complexity of enqueue operation using a linked list with rear pointer?

A O(1)

B O(n)

C O(log n)

D O(n^2)

Enqueue is O(1) because we add the new node directly at the rear pointer.

If the queue has elements 5 -> 10 -> 15, what will be the queue after enqueueing 20?

A 20 -> 5 -> 10 -> 15

B 5 -> 10 -> 15 -> 20

C 5 -> 10 -> 20 -> 15

D 10 -> 15 -> 20

Enqueue adds the new element at the end: 5 -> 10 -> 15 -> 20.

Explain step-by-step how to enqueue a new element in a queue implemented with a linked list.

Think about what happens when the queue is empty versus when it has elements.

Describe the state of the queue pointers and nodes after enqueueing multiple elements using a linked list.

Visualize the linked list as a chain of nodes from front to rear.

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