Most Commonly Asked Data Structure Interview Questions on Deque

A deque (double-ended queue) is a powerful linear data structure that allows insertion and deletion from both ends. It is widely used in real-world applications like sliding window problems, task scheduling, and undo operations.

1. What are the main operations of a deque?

The main operations include:

• addFront/pushFront: Insert an element at the front.
• addRear/pushBack: Insert an element at the rear.
• removeFront/popFront: Remove an element from the front.
• removeRear/popBack: Remove an element from the rear.
• peekFront/front: View the front element without removing it.
• peekRear/rear: View the rear element without removing it.
• isEmpty: Check if the deque is empty.
• size: Get the number of elements.

2. How is a deque implemented?

A deque can be implemented in three ways:

• Array-based deque: Can use a fixed-size or dynamic array. Circular indexing is often used to optimize operations at both ends.
• Linked List-based deque: Uses a doubly linked list, allowing O(1) insertion and deletion at both ends.
• Library-based deque: Built-in implementations handle resizing and memory management internally.

3. What is the time complexity of deque operations?

• Insertion and deletion at both ends take O(1) in both array-based (with circular indexing) and linked list-based implementations.
• Accessing the front or rear element also takes O(1).
• Searching for a specific element in a deque takes O(n) time.

4. How does a deque differ from a queue and a stack?

A deque differs from a queue and a stack because it allows insertion and deletion from both ends, while a queue and a stack restrict operations to specific ends.

• Queue – insertion at the rear, deletion at the front (FIFO).
• Stack – insertion and deletion at the same end (LIFO).
• Deque – insertion and deletion at both front and rear.

5. Explain the concept of a circular deque?

A circular deque is a double-ended queue implemented in a circular manner using an array. When either end reaches the boundary, it wraps around to the other end, ensuring efficient use of space. It supports constant-time insertions and deletions at both front and rear and is widely used in buffering and scheduling.

6. What is a monotonic deque, and how is it used?

A monotonic deque is a deque that maintains elements in either increasing or decreasing order. It is commonly used in sliding window problems to efficiently track the minimum or maximum.

• For maximum: push new elements at the rear while removing smaller ones, keeping the deque in decreasing order.
• The front always holds the maximum of the current window.
• Remove elements from the front if they move out of the window.

This ensures each element is processed at most twice, achieving O(n) time complexity.

7. How is a deque different from a doubly linked list?

• Deque: An abstract data type (ADT) that defines a double-ended queue with operations like addFront, removeRear, etc. It focuses on the interface and behavior (insert/delete at both ends) and can be implemented using various structures.
• Doubly Linked List: A concrete data structure where each node has pointers to both the next and previous nodes. It is one possible implementation of a deque, providing O(1) operations at both ends but can also support other operations (e.g., inserting at arbitrary positions).

A deque defines what operations are needed, while a doubly linked list is how those operations can be implemented.

8. How would you use a deque to check for palindromes efficiently?

To check if a sequence (e.g., a string) is palindrome using a deque:

• Push all characters into the deque (addRear).
• Repeatedly compare and remove the front and rear elements using removeFront and removeRear.
• If all pairs match until the deque is empty (or has one element), the sequence is a palindrome. This leverages the deque’s O(1) access to both ends achieving O(n) time complexity with minimal memory overhead.

9. What are the applications of deques?

Deques are used in various real world and algorithmic scenarios, including:

• Sliding Window Problems: Efficiently track maximum/minimum in a window of size k.
• Task Scheduling: Insert high-priority tasks at the front.
• Undo/Redo Operations: Maintain history with operations at both ends.
• Palindrome Checking: Compare characters from both ends efficiently.
• Cache Implementation: Used in LRU caches to manage items.

Top Problems on Deque Data Structure

Below is a list of top deque related coding problems ranging from easy to hard, commonly asked in software development engineer (SDE) interviews.

Top Problems on Deque data structure