Dequeue Operation in DSA Typescript - Execution Trace

Concept Flow - Dequeue Operation

Check if Deque is empty

No↓

Choose operation: Remove Front or Remove Rear

↓

Remove Front

↓

Update front pointer

↓

Return removed element

This flow shows how a dequeue operation removes an element from either the front or rear, updating pointers accordingly.

Execution Sample

DSA Typescript

deque = [10, 20, 30, 40]
removeFront()
removeRear()

Removes one element from the front and one from the rear of the deque.

Execution Table

Step	Operation	Nodes in Deque	Pointer Changes	Visual State
1	Initial Deque	[10, 20, 30, 40]	front -> 10, rear -> 40	10 <-> 20 <-> 30 <-> 40
2	removeFront()	[20, 30, 40]	front moves from 10 to 20	20 <-> 30 <-> 40
3	removeRear()	[20, 30]	rear moves from 40 to 30	20 <-> 30
4	End	[20, 30]	front -> 20, rear -> 30	20 <-> 30

💡 No more operations; deque updated after removals.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
front	10	20	20	20
rear	40	40	30	30
deque	[10, 20, 30, 40]	[20, 30, 40]	[20, 30]	[20, 30]

Key Moments - 3 Insights

Why does the front pointer move to the next node after removeFront?

Because the front node is removed, the front pointer must update to the next node to keep the deque valid, as shown in step 2 of the execution_table.

What happens if we try to remove from an empty deque?

The operation stops immediately since the deque is empty, preventing pointer updates or errors. This is the first check in the concept_flow.

Why does the rear pointer move backward after removeRear?

Removing from the rear means the rear pointer must move to the previous node to reflect the new end of the deque, as shown in step 3 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the new front pointer value?

A 10

B 20

C 30

D 40

💡 Hint

Check the 'Pointer Changes' column at step 2 in the execution_table.

At which step does the rear pointer move from 40 to 30?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Pointer Changes' column for rear pointer updates.

If we remove another front element after step 3, what would be the deque state?

A [30]

B [20]

C [40]

D []

💡 Hint

Check the 'Nodes in Deque' column and imagine removing front again.

Concept Snapshot

Dequeue Operation:
- Removes element from front or rear
- Update front or rear pointer accordingly
- Check if deque is empty before removal
- After removal, pointers reflect new ends
- Visual: 10 <-> 20 <-> 30 <-> 40
  removeFront() → 20 <-> 30 <-> 40
  removeRear() → 20 <-> 30

Full Transcript

A dequeue operation removes an element from either the front or rear of the deque. First, it checks if the deque is empty to avoid errors. If removing from the front, the front pointer moves to the next node, effectively removing the first element. If removing from the rear, the rear pointer moves to the previous node, removing the last element. The deque's state updates after each removal, maintaining the linked structure. For example, starting with [10, 20, 30, 40], removing front gives [20, 30, 40], then removing rear gives [20, 30].

Practice

1. What is the main difference between a dequeue and a queue?

easy

A. A dequeue allows insertion and deletion from both ends.

B. A dequeue only allows insertion at the rear.

C. A dequeue only allows deletion from the front.

D. A dequeue is a fixed size data structure.

Solution

1. Step 1: Understand queue operations

   A queue allows insertion at the rear and deletion from the front only.

2. Step 2: Understand dequeue operations

   A dequeue allows insertion and deletion at both front and rear ends.

3. Final Answer:

   A dequeue allows insertion and deletion from both ends. -> Option A

4. Quick Check:

   Dequeue = Both ends operations [OK]

Hint: Dequeue = Double-ended queue, both ends allowed [OK]

Common Mistakes:

• Thinking dequeue is same as queue
• Assuming insertion only at rear
• Confusing dequeue with stack

2. Which of the following is the correct TypeScript method to add an element at the front of a dequeue implemented as an array?

easy

A. dequeue.pushFront(element);

B. dequeue.unshift(element);

C. dequeue.addFirst(element);

D. dequeue.insertFront(element);

Solution

1. Step 1: Recall array methods for front insertion

   In TypeScript/JavaScript, unshift() adds element at the front of an array.

2. Step 2: Check other options

   Methods like pushFront, addFirst, insertFront do not exist in standard arrays.

3. Final Answer:

   dequeue.unshift(element); -> Option B

4. Quick Check:

   Array front insert = unshift() [OK]

Hint: Use unshift() to add at front in arrays [OK]

Common Mistakes:

• Using push() instead of unshift()
• Assuming pushFront() exists
• Confusing with linked list methods

3. Consider this TypeScript code using an array as a dequeue:

let dequeue = [10, 20, 30];
dequeue.pop();
dequeue.unshift(5);
console.log(dequeue);

What is the output?

medium

A. [5, 10, 20]

B. [5, 10, 20, 30]

C. [10, 20, 30, 5]

D. [10, 20]

Solution

1. Step 1: Execute pop()

   pop() removes last element 30, so array becomes [10, 20].

2. Step 2: Execute unshift(5)

   unshift(5) adds 5 at front, array becomes [5, 10, 20].

3. Final Answer:

   [5, 10, 20] -> Option A

4. Quick Check:

   pop removes last, unshift adds front [OK]

Hint: pop removes last, unshift adds front [OK]

Common Mistakes:

• Assuming pop removes front
• Thinking unshift adds at end
• Confusing array order after operations

4. The following TypeScript code tries to remove an element from the front of a dequeue implemented as an array:

let dequeue = [1, 2, 3];
dequeue.removeFront();
console.log(dequeue);

What is the issue and how to fix it?

medium

A. removeFront() works only on linked lists; convert array to linked list.

B. removeFront() removes last element; use pop() instead.

C. removeFront() is not a valid method; use shift() instead.

D. No issue; code runs correctly.

Solution

1. Step 1: Identify invalid method

   Arrays in TypeScript do not have removeFront() method; this causes a runtime error.

2. Step 2: Use correct method for front removal

   shift() removes the first element from an array correctly.

3. Final Answer:

   removeFront() is not a valid method; use shift() instead. -> Option C

4. Quick Check:

   Front removal in arrays = shift() [OK]

Hint: Use shift() to remove front element in arrays [OK]

Common Mistakes:

• Using non-existent removeFront() method
• Confusing pop() with front removal
• Thinking arrays have linked list methods

5. You have a dequeue implemented as an array: let dq = [1, 2, 3, 4];. You perform these operations:

dq.pop();
dq.unshift(0);
dq.push(5);
dq.shift();
console.log(dq);

What is the final state of the dequeue?

hard

A. [0, 1, 2, 3, 5]

B. [2, 3, 4, 5]

C. [1, 2, 3, 4, 5]

D. [1, 2, 3, 5]

Solution

1. Step 1: Execute pop()

   pop() removes last element 4, dq becomes [1, 2, 3].

2. Step 2: Execute unshift(0)

   unshift(0) adds 0 at front, dq becomes [0, 1, 2, 3].

3. Step 3: Execute push(5)

   push(5) adds 5 at end, dq becomes [0, 1, 2, 3, 5].

4. Step 4: Execute shift()

   shift() removes first element 0, dq becomes [1, 2, 3, 5].

5. Final Answer:

   [1, 2, 3, 5] -> Option D

6. Quick Check:

   pop removes end, unshift adds front, push adds end, shift removes front [OK]

Hint: Track each operation stepwise on array [OK]

Common Mistakes:

• Forgetting shift removes front element
• Mixing order of operations
• Assuming unshift adds at end