20+ Most Asked Data Structures Interview Questions & Answers

Data Structures & Algorithms are among the most frequently tested topics in both IT service-based and product-based company interviews. Mastering these concepts is crucial for cracking technical interviews.

In this article, we’ll explore over 20 of the most commonly asked data structures interview questions, along with detailed answers.

What is a Data Structure?

A data structure is a method of organizing and storing data efficiently. It is not a programming language but an implementation concept that can be applied using languages like C, C++, Java, and Python.

Example: Data such as photos and videos are stored in a gallery using a data structure.

Applications of Data Structures

Here are some real-world applications of data structures:

• Telephone Networks: Representing city telephone networks.
• Web Browsers: Implementing the “Back” functionality.
• Dynamic Storage: Storing frequently accessed data using Hash Tables.
• Undo Functionality: Implementing undo in text editors using Stacks.
• File Management: Storing directory and file structures in an operating system.

Key Differences: Linked List vs. Array

Why use a Linked List over an Array?

When storing a large dataset, if contiguous memory is not available, arrays become inefficient. Linked Lists overcome this by using pointers.

Creating a Node in a Singly Linked List (C Code)

struct node {
    int data;
    struct node *next;
};

struct node *head, *ptr;
ptr = (struct node *)malloc(sizeof(struct node));

Doubly Linked List vs. Singly Linked List

A Doubly Linked List has both forward and backward links, allowing bidirectional traversal. In contrast, a Singly Linked List only has forward links.

Array vs. Stack

Stack Characteristics:

• Dynamic size
• Stores elements of different data types

Array Characteristics:

• Fixed size
• Stores elements of the same data type

Implementing Priority Queue with Minimum Queues

At least two queues are required:

1. One for storing data.
2. Another for maintaining priorities.

Tree Traversal Techniques

There are three main tree traversal methods:

1. In-order: Left -> Root -> Right
2. Pre-order: Root -> Left -> Right
3. Post-order: Left -> Right -> Root

Why is Searching in a Binary Search Tree Efficient?

A Binary Search Tree (BST) allows searching in O(log n) time by recursively checking left or right subtrees based on value comparison.

Applications of Graphs

Graphs are used in:

• Circuit Networks (Vertices = connection points, Edges = wires)
• Transport Networks (Vertices = stations, Edges = routes)
• Maps (Vertices = cities, Edges = roads)
• Program Flow Analysis (Vertices = modules, Edges = function calls)

Can We Use Binary Search on a Sorted Linked List?

No, because binary search requires O(1) index access, which linked lists do not provide.

Memory Leaks

A memory leak occurs when dynamically allocated memory is not freed after use.

Checking If a Binary Tree is a BST

Use in-order traversal. If the output is sorted, then the tree is a BST.

Best Data Structure for Recursion

The Stack is ideal for recursion due to its LIFO (Last In, First Out) property.

Convert Infix Expression to Prefix and Postfix

Given: ((A + B) * C - (D - E) ^ (F + G))

• Prefix: ^ * +ABC DE + FG
• Postfix: AB + C * DE FG + ^

Important Data Structures Interview Programs

Here are some essential coding problems for data structure interviews:

1. Sorting a Stack Using Another Stack
2. Finding the Nth Node from End in a Linked List
3. Replacing Each Element by Its Rank in an Array
4. Checking if a Graph is a Tree
5. Finding the Kth Smallest Element in an Unsorted Array
6. Finding the Shortest Path Between Two Vertices in a Graph

Final Thoughts

Mastering data structures is crucial for acing technical interviews. Make sure to practice implementation problems and understand the underlying concepts.

Stay ahead by exploring more Data Structure & Algorithm interview questions!