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.

Practice

1. What is a deque in Python's collections module?

easy

A. A list that automatically sorts elements when inserted

B. A queue that only allows insertion at the rear and removal from the front

C. A stack that only allows insertion and removal from the top

D. A double-ended queue allowing insertion and removal from both ends

Solution

1. Step 1: Understand the deque concept

   A deque is a data structure that supports adding and removing elements from both the front and the back efficiently.

2. Step 2: Compare with other data structures

   Unlike a queue or stack, deque allows operations on both ends, not just one end.

3. Final Answer:

   A double-ended queue allowing insertion and removal from both ends -> Option D

4. Quick Check:

   Deque = double-ended queue [OK]

Hint: Remember: deque means double-ended queue [OK]

Common Mistakes:

• Confusing deque with stack or queue
• Thinking deque only works like a queue
• Assuming deque sorts elements automatically

2. Which of the following is the correct way to import and create an empty deque in Python?

easy

A. import collections\nd = collections.deque[]

B. from collections import deque\nd = deque()

C. from collections import Deque\nd = Deque()

D. import deque\nd = deque()

Solution

1. Step 1: Check the correct import statement

   The deque class is in the collections module and must be imported as from collections import deque.

2. Step 2: Verify deque creation syntax

   Creating an empty deque is done by calling deque() with parentheses, not brackets.

3. Final Answer:

   from collections import deque\nd = deque() -> Option B

4. Quick Check:

   Correct import and empty deque creation [OK]

Hint: Use 'from collections import deque' and call deque() [OK]

Common Mistakes:

• Using uppercase 'Deque' instead of 'deque'
• Trying to import deque directly without collections
• Using square brackets [] instead of parentheses ()

3. What will be the output of the following code?

from collections import deque
d = deque([1, 2, 3])
d.appendleft(0)
d.append(4)
print(list(d))

medium

A. [4, 1, 2, 3, 0]

B. [1, 2, 3, 0, 4]

C. [0, 1, 2, 3, 4]

D. [1, 2, 3, 4, 0]

Solution

1. Step 1: Trace deque initialization and appendleft

   Deque starts as [1, 2, 3]. After appendleft(0), it becomes [0, 1, 2, 3].

2. Step 2: Trace append operation

   After append(4), 4 is added to the right end, making deque [0, 1, 2, 3, 4].

3. Final Answer:

   [0, 1, 2, 3, 4] -> Option C

4. Quick Check:

   appendleft adds front, append adds end [OK]

Hint: appendleft adds front, append adds end [OK]

Common Mistakes:

• Confusing appendleft with append
• Assuming append adds to front
• Not converting deque to list before printing

4. Find the error in the following code snippet:

from collections import deque
d = deque()
d.appendleft(1)
d.append(2)
d.pop()
d.pop()
d.pop()

medium

A. Calling pop() on an empty deque causes an IndexError

B. appendleft() is not a valid method for deque

C. deque() cannot be initialized empty

D. append() requires two arguments

Solution

1. Step 1: Analyze deque operations

   Deque starts empty, then 1 is added to front, 2 to end. After two pops, deque is empty.

2. Step 2: Identify error on third pop

   The third pop tries to remove from empty deque, causing IndexError.

3. Final Answer:

   Calling pop() on an empty deque causes an IndexError -> Option A

4. Quick Check:

   pop on empty deque throws error [OK]

Hint: pop() on empty deque causes error [OK]

Common Mistakes:

• Thinking appendleft() is invalid
• Assuming deque can't be empty
• Believing append() needs two arguments

5. You have a deque initialized as d = deque([10, 20, 30, 40]). You want to rotate it so that the element 30 becomes the first element. Which code snippet achieves this correctly?

hard

A. d.rotate(-2)

B. d.rotate(2)

C. d.rotate(-3)

D. d.rotate(3)

Solution

1. Step 1: Understand deque rotation

   Positive rotate moves elements to the right; negative rotate moves to the left.

2. Step 2: Calculate rotation to bring 30 to front

   30 is at index 2. Rotating left by 2 (rotate(-2)) moves 30 to front.

3. Final Answer:

   d.rotate(-2) -> Option A

4. Quick Check:

   Negative rotate shifts left, bringing 30 front [OK]

Hint: Negative rotate shifts left; positive shifts right [OK]

Common Mistakes:

• Using positive rotate instead of negative
• Confusing rotation direction
• Not counting index positions correctly