Double Ended Queue Deque in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Deque Operations

What is the printed state of the deque after executing the following Python code?

DSA Python

from collections import deque

d = deque()
d.append(1)
d.appendleft(2)
d.append(3)
d.pop()
d.appendleft(4)
print(list(d))

A [2, 4, 1]

B [4, 2, 1]

C [2, 1, 4]

D [4, 1, 2]

💡 Hint

Remember that appendleft adds to the front and pop removes from the end.

The operations modify the deque as follows:

• append(1): deque is [1]
• appendleft(2): deque is [2, 1]
• append(3): deque is [2, 1, 3]
• pop(): removes 3, deque is [2, 1]
• appendleft(4): deque is [4, 2, 1]

🧠 Conceptual

intermediate

Deque Characteristics

Which of the following statements about a double-ended queue (deque) is TRUE?

A Elements can only be added or removed from the front.

B Deque is a fixed-size data structure and cannot grow dynamically.

C Elements can be added or removed from both front and rear ends.

D Deque only supports FIFO (First In First Out) operations.

💡 Hint

Think about what 'double-ended' means.

A deque allows insertion and deletion at both ends, front and rear, making option C correct.

Option C is false because operations are allowed at both ends.

Option C is false because deque supports both FIFO and LIFO behaviors.

Option C is false because deque can grow dynamically.

🔧 Debug

advanced

Identify the Error in Deque Usage

What error will the following code produce when executed?

DSA Python

from collections import deque

d = deque([1, 2, 3])
d.pop()
d.pop()
d.pop()
d.pop()

A TypeError: pop() takes no arguments

B AttributeError: 'deque' object has no attribute 'pop'

C No error, prints an empty deque

D IndexError: pop from an empty deque

💡 Hint

Consider what happens when you pop more elements than present.

The deque initially has 3 elements. Popping 4 times tries to remove an element from an empty deque on the last pop, causing an IndexError.

❓ Predict Output

advanced

Deque Rotation Result

What is the output of the following code snippet?

DSA Python

from collections import deque

d = deque([10, 20, 30, 40, 50])
d.rotate(2)
print(list(d))

A [40, 50, 10, 20, 30]

B [30, 40, 50, 10, 20]

C [50, 10, 20, 30, 40]

D [20, 30, 40, 50, 10]

💡 Hint

Rotation moves elements from the end to the front when positive.

Rotating by 2 moves the last two elements (40, 50) to the front, resulting in [40, 50, 10, 20, 30].

🚀 Application

expert

Deque Usage in Sliding Window Maximum

Given a list of integers and a window size k, a deque is used to find the maximum in each sliding window of size k. Which property of the deque is MOST IMPORTANT for this algorithm to work efficiently?

A Ability to add and remove elements from both ends in O(1) time.

B Ability to add and remove elements only from the rear end.

C Ability to sort elements automatically inside the deque.

D Ability to access elements by index in O(1) time.

💡 Hint

Think about how the sliding window moves and how elements are added or removed.

The sliding window maximum algorithm requires adding and removing elements from both ends efficiently to maintain candidates for maximum values.

Deque supports O(1) insertion and deletion at both ends, which is crucial.

It does not sort elements automatically, nor does it provide O(1) random access.