Double Ended Queue Deque in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Double Ended Queue (Deque)?

A Double Ended Queue, or Deque, is a data structure where elements can be added or removed from both the front and the back.

beginner

Name two main operations you can perform on a Deque.

You can add elements (called enqueue) and remove elements (called dequeue) from both the front and the back ends.

beginner

How does a Deque differ from a regular Queue?

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

intermediate

What is the output after these operations on an empty Deque: add_front(1), add_back(2), remove_front(), add_back(3)?

After add_front(1): 1<br>After add_back(2): 1 -> 2<br>After remove_front(): 2<br>After add_back(3): 2 -> 3

beginner

Why is a Deque useful in real life?

Because it lets you add or remove items from both ends, it is like a line where people can enter or leave from front or back, useful for tasks like undo/redo or sliding windows.

Which operation is NOT possible in a Deque?

A Add element at front

B Add element in the middle

C Remove element from back

D Remove element from front

Deques allow adding/removing only at front or back, not in the middle.

What is the state of the Deque after add_back(5) on an empty Deque?

A Error

B Empty

C null

D 5

Adding 5 at back on empty Deque results in Deque with one element: 5.

If a Deque has elements 3 -> 4 -> 5, what is the result after remove_back()?

A 3 -> 4

B 4 -> 5

C 3 -> 5

D Empty

Removing from back removes 5, leaving 3 -> 4.

Which real-life example best matches a Deque?

A A line where people can enter or leave from front or back

B A stack of plates where you add/remove only from top

C A line where people enter only at the back

D A random pile of books

Deque allows adding/removing from both ends, like a line with two open doors.

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

A O(n)

B O(log n)

C O(1)

D O(n^2)

Deque operations at front or back are done in constant time O(1).

Explain what a Double Ended Queue (Deque) is and how it works.

Think about a line where people can enter or leave from front or back.

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

Consider undo/redo or sliding window problems.