Dequeue Operation in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a dequeue in data structures?

A dequeue (double-ended queue) is a data structure where elements can be added or removed from both the front and the rear ends.

beginner

Name the two main operations to remove elements in a dequeue.

The two main removal operations are dequeueFront (removes from the front) and dequeueRear (removes from the rear).

beginner

What happens if you try to dequeue from an empty dequeue?

Trying to dequeue from an empty dequeue results in an underflow condition, meaning there are no elements to remove.

intermediate

Explain the difference between dequeueFront and dequeueRear operations.

dequeueFront removes the element at the front end of the dequeue, while dequeueRear removes the element at the rear end. Both operations reduce the size of the dequeue by one.

intermediate

Why is a dequeue useful compared to a simple queue?

A dequeue is more flexible because it allows insertion and deletion at both ends, unlike a simple queue which only allows these operations at one end.

Which operation removes an element from the rear end of a dequeue?

A enqueueRear

B dequeueFront

C enqueueFront

D dequeueRear

dequeueRear removes an element from the rear end of the dequeue.

What is the result of dequeueFront on an empty dequeue?

A Removes the last element

B Underflow error or no operation

C Adds an element

D Returns the size of dequeue

DequeueFront on an empty dequeue causes underflow because there are no elements to remove.

Which of the following is NOT a valid dequeue operation?

A dequeueMiddle

B enqueueRear

C enqueueFront

D dequeueFront

DequeueMiddle is not a standard operation; dequeue operations happen only at front or rear.

In a dequeue, where can elements be added?

A Only at the front

B Only at the rear

C At both front and rear

D Only in the middle

Elements can be added at both the front and rear ends in a dequeue.

What data structure is a dequeue most similar to?

A Queue

B Binary Tree

C Linked List

D Stack

A dequeue is a generalized form of a queue allowing operations at both ends.

Describe the dequeue operation and its two main removal methods.

Think about how elements are removed from both ends.

Explain why a dequeue is more flexible than a simple queue.

Consider where you can add or remove elements.

Practice

1. What is the main feature of a deque (double-ended queue) compared to a regular queue?

easy

A. It only stores unique elements

B. Elements can only be added at the rear

C. Elements can be added or removed from both ends

D. Elements can only be removed from the front

Solution

1. Step 1: Understand queue vs deque

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

2. Step 2: Identify deque feature

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

3. Final Answer:

   Elements can be added or removed from both ends -> Option C

4. Quick Check:

   Deque = double-ended queue [OK]

Hint: Deque means double-ended, so both ends work [OK]

Common Mistakes:

• Thinking deque works like a normal queue
• Assuming elements can only be added at one end
• Confusing deque with stack

2. Which of the following is the correct syntax to add an element '5' to the front of a deque named d in Python?

easy

A. d.push_front(5)

B. d.addfront(5)

C. d.insert_front(5)

D. d.appendleft(5)

Solution

1. Step 1: Recall deque methods in Python

   Python's collections.deque uses appendleft() to add at front.

2. Step 2: Check method names

   Other options are not valid deque methods in Python.

3. Final Answer:

   d.appendleft(5) -> Option D

4. Quick Check:

   appendleft adds to front [OK]

Hint: Use appendleft() to add front in Python deque [OK]

Common Mistakes:

• Using non-existent methods like push_front
• Confusing append() with appendleft()
• Trying to use list methods on deque

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

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

medium

A. [0, 1, 2]

B. [1, 2, 3, 0]

C. [0, 1, 2, 3]

D. [1, 2]

Solution

1. Step 1: Initial deque and appendleft

   Start with deque [1, 2, 3]. appendleft(0) adds 0 at front: [0, 1, 2, 3].

2. Step 2: pop removes from rear

   pop() removes last element 3: [0, 1, 2].

3. Final Answer:

   [0, 1, 2] -> Option A

4. Quick Check:

   appendleft + pop = [0,1,2] [OK]

Hint: appendleft adds front, pop removes rear [OK]

Common Mistakes:

• Thinking pop removes from front
• Ignoring appendleft effect
• Confusing list(d) output

4. Identify the error in the following code snippet:

from collections import deque
d = deque([10, 20, 30])
d.removeleft()
print(d)

medium

A. removeleft() is not a valid deque method

B. deque cannot be initialized with a list

C. print(d) should be print(list(d))

D. No error, code runs fine

Solution

1. Step 1: Check deque methods

   deque has no method named removeleft(); correct method to remove front is popleft().

2. Step 2: Validate other parts

   deque can be initialized with list; print(d) works; so no other errors.

3. Final Answer:

   removeleft() is not a valid deque method -> Option A

4. Quick Check:

   Use popleft() to remove front [OK]

Hint: Use popleft(), not removeleft() [OK]

Common Mistakes:

• Using non-existent removeleft() method
• Thinking print(d) causes error
• Confusing initialization syntax

5. You have a deque d = deque([1, 2, 3, 4]). You perform these operations:

d.pop()
d.appendleft(0)
d.popleft()
d.append(5)
print(list(d))

What is the final state of the deque?

hard

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

B. [1, 2, 3, 5]

C. [2, 3, 4, 5]

D. [0, 2, 3, 5]

Solution

1. Step 1: Trace each operation on deque

   Start: [1, 2, 3, 4]
   d.pop() removes 4: [1, 2, 3]
   d.appendleft(0): [0, 1, 2, 3]
   d.popleft() removes 0: [1, 2, 3]
   d.append(5): [1, 2, 3, 5]

2. Step 2: Confirm final deque state

   After all operations, deque is [1, 2, 3, 5].

3. Final Answer:

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

4. Quick Check:

   pop + appendleft + popleft + append = [1,2,3,5] [OK]

Hint: Apply operations step-by-step on deque [OK]

Common Mistakes:

• Forgetting popleft removes front
• Mixing order of operations
• Assuming appendleft stays after popleft