Deque (double-ended queue) in Data Structures Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Deque Operations

Which of the following statements correctly describes the behavior of a deque?

A A deque allows insertion and removal of elements only at the front end.

B A deque allows insertion and removal of elements only at the rear end.

C A deque allows insertion and removal of elements at both the front and rear ends.

D A deque allows insertion at the front and removal only from the rear.

💡 Hint

Think about what 'double-ended' means in the name.

A deque (double-ended queue) supports adding and removing elements from both ends, front and rear.

📋 Factual

intermediate

Deque Implementation Characteristics

Which data structure is most commonly used to efficiently implement a deque?

A Singly linked list

B Doubly linked list

C Binary tree

D Stack

💡 Hint

Consider which structure allows easy access at both ends.

Doubly linked lists allow efficient insertion and deletion at both ends, making them ideal for deques.

🔍 Analysis

advanced

Deque Operation Time Complexity

What is the average time complexity for insertion and deletion operations at both ends of a deque implemented with a doubly linked list?

A O(1) for both insertion and deletion

B O(n) for insertion and O(1) for deletion

C O(1) for insertion and O(n) for deletion

D O(n) for both insertion and deletion

💡 Hint

Think about how doubly linked lists allow direct access to both ends.

Insertion and deletion at both ends in a doubly linked list are done in constant time, O(1).

❓ Comparison

advanced

Deque vs Queue Behavior

Which of the following best describes a key difference between a deque and a standard queue?

A A deque allows insertion and removal at both ends, while a queue allows insertion at rear and removal at front only.

B A deque only allows insertion at the rear, while a queue allows insertion at both ends.

C A deque allows removal only at the rear, while a queue allows removal at both ends.

D A deque and a queue have identical insertion and removal rules.

💡 Hint

Recall the basic rules of a queue compared to a deque.

A queue inserts at the rear and removes from the front only, while a deque supports both ends for insertion and removal.

❓ Reasoning

expert

Deque Usage in Real-World Applications

Which scenario best demonstrates an ideal use case for a deque?

A Processing tasks strictly in the order they arrive, without reordering.

B Storing data where only the last inserted item is accessed frequently.

C Managing a fixed-size buffer where only the oldest data is removed.

D Implementing undo and redo functionality where actions can be added or removed from both ends.

💡 Hint

Think about situations where you need to add or remove items from both ends.

Undo and redo operations require adding and removing actions from both ends, which a deque supports efficiently.

Practice

1. What is a key feature of a deque compared to a regular queue?

easy

A. It allows adding and removing elements from both ends.

B. It only allows adding elements at the rear.

C. It only allows removing elements from the front.

D. It stores elements in sorted order automatically.

Solution

1. Step 1: Understand queue vs deque

   A regular queue allows adding at the rear and removing from the front only.

2. Step 2: Identify deque's special feature

   A deque allows both adding and removing elements from front and rear ends.

3. Final Answer:

   It allows adding and removing elements from both ends. -> Option A

4. Quick Check:

   Deque = double-ended queue = both ends operations [OK]

Hint: Deque means double-ended: both front and rear operations [OK]

Common Mistakes:

• Thinking deque behaves exactly like a queue
• Assuming deque only adds at rear
• Confusing deque with stack

2. Which of the following is the correct operation to add an element at the front of a deque?

easy

A. push(element)

B. add_front(element)

C. enqueue(element)

D. insert_end(element)

Solution

1. Step 1: Recall deque operations

   Common deque operations include add_front, add_rear, remove_front, remove_rear.

2. Step 2: Match operation to adding at front

   The operation to add at front is named add_front(element).

3. Final Answer:

   add_front(element) -> Option B

4. Quick Check:

   Add front = add_front() method [OK]

Hint: Add front uses add_front(), not enqueue or push [OK]

Common Mistakes:

• Confusing enqueue with add_front
• Using push which is stack operation
• Assuming insert_end adds at front

3. Consider this sequence of operations on an empty deque:
add_rear(10)
add_front(20)
add_rear(30)
remove_front()
What is the element removed by remove_front()?

medium

A. 20

B. 10

C. 30

D. None, deque is empty

Solution

1. Step 1: Track elements after each operation

   Start empty. add_rear(10) -> deque: [10]. add_front(20) -> deque: [20, 10]. add_rear(30) -> deque: [20, 10, 30].

2. Step 2: Identify element removed by remove_front()

   remove_front() removes from front, which is 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   Front element removed = 20 [OK]

Hint: Track deque ends step-by-step to find removed element [OK]

Common Mistakes:

• Removing from rear instead of front
• Confusing order after add_front and add_rear
• Assuming remove_front removes last added element

4. A programmer wrote this code snippet for a deque:
deque = []
deque.add_front(5)
deque.add_rear(10)
What is the error in this code?

medium

A. No error, code is correct

B. add_rear should be add_back

C. List object has no method add_front

D. Deque must be imported from collections

Solution

1. Step 1: Check data structure type and methods

   Python list does not have add_front or add_rear methods.

2. Step 2: Identify correct usage for deque

   Deque methods like appendleft() and append() exist in collections.deque, not in list.

3. Final Answer:

   List object has no method add_front -> Option C

4. Quick Check:

   List lacks add_front method error [OK]

Hint: List has no add_front; use collections.deque for deque methods [OK]

Common Mistakes:

• Assuming list supports deque methods
• Thinking add_rear is named add_back
• Ignoring need to import deque class

5. You want to implement an undo feature using a deque. Which deque operation sequence best supports undoing the last action?

hard

A. add_front(action), remove_rear() to undo

B. add_front(action), remove_front() to undo

C. add_rear(action), remove_front() to undo

D. add_rear(action), remove_rear() to undo

Solution

1. Step 1: Understand undo behavior

   Undo removes the most recent action added, so last-in-first-out behavior is needed.

2. Step 2: Match deque operations to LIFO

   Adding actions at rear and removing from rear simulates stack behavior for undo.

3. Final Answer:

   add_rear(action), remove_rear() to undo -> Option D

4. Quick Check:

   Undo = last action removed from rear [OK]

Hint: Undo removes last action added at rear using remove_rear() [OK]

Common Mistakes:

• Removing from front instead of rear for undo
• Adding actions at front but removing from front
• Confusing undo with queue FIFO behavior