Deque (double-ended queue) in Data Structures Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Deque (double-ended queue)?

A Deque is a data structure where elements can be added or removed from both the front and the back. It combines features of both stacks and queues.

beginner

Name two main operations you can perform on a Deque.

You can add elements to the front or back (called 'push front' and 'push back') and remove elements from the front or back (called 'pop front' and 'pop back').

beginner

How is a Deque different from a regular queue?

A regular queue only allows adding at the back and removing from the front. A Deque allows adding and removing from both ends.

intermediate

Give a real-life example where a Deque might be useful.

A Deque can be used in a browser's history feature, where you can go back (pop from the back) or forward (pop from the front) through pages.

intermediate

What is the time complexity of adding or removing elements from a Deque?

Adding or removing elements from either end of a Deque usually takes constant time, meaning it is very fast and does not depend on the number of elements.

Which ends of a Deque can you add elements to?

A Only front

B Both front and back

C Only back

D Neither end

A Deque allows adding elements at both the front and the back.

What does 'pop front' mean in a Deque?

A Add an element to the back

B Add an element to the front

C Remove an element from the back

D Remove an element from the front

'Pop front' means removing an element from the front of the Deque.

Which data structure is a Deque most similar to?

A Graph

B Tree

C Stack and Queue combined

D Hash Table

A Deque combines features of both stacks (LIFO) and queues (FIFO).

If you can only add elements at the back and remove from the front, what data structure do you have?

A Queue

B Deque

C Stack

D Array

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

What is the typical time complexity for adding or removing elements at either end of a Deque?

A Constant time (O(1))

B Linear time (O(n))

C Logarithmic time (O(log n))

D Quadratic time (O(n²))

Operations at either end of a Deque usually take constant time, meaning they are very fast.

Explain what a Deque is and how it differs from a regular queue.

Think about where you can add or remove elements.

Describe a real-life situation where using a Deque would be helpful.

Consider situations where you might go forward and backward.

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