Data Structure Interview Questions to Check Out in 2021

Data Structure Interview Questions to Check Out in 2021
Data Structure Interview Questions to Check Out in 2021


Good preparation is required for a successful interview. You never know what questions the interviewer can ask you in an interview. They test the knowledge of the candidate and assess their experience. You can answer these questions if you have a deep understanding of the topic.

Data structure interview questions are very crucial to prepare for a Java interview or a Data Science interview. Therefore, sound knowledge of Data Structures and Algorithms is a must before you go for such an interview.

The Data Structure interview questions are the most common interview questions asked in an interview. These questions will help you in revising the concepts also. You should go through these Data Structure interview questions before you go for your interview.

What is a Data Structure?

A data structure is a method to organise the data to be used more effectively. Different types of data structures suit different applications or specific tasks. As an example, hash tables have the application in compiler implementations.

Types of Data Structure

There are two types of data structure:

  1. Linear: A linear data structure is one where the elements are present as a linear list or a sequence. For example, array, stack, linked list, queue.
  2. Non-Linear: A non-linear data structure is one in which the nodes need to be traversed nonlinearly. For example, tree and graph.

Characteristics of Data Structure

Data structures have three main characteristics:

  1. Correctness: The implementation of the interface in the data structure should be correct.
  2. Time complexity: For the operations of data structures, the execution or running time should be as short as possible.
  3. Space Complexity: Data structure should use as minimal memory as possible.

Data Structure Interview Questions

Read the below Data Structure interview questions before appearing for your upcoming interview:

1. What are the commonly used data structure operations?

Below are the set of operations that any of the data structure can perform:

  • Insertion: It means adding an element to the data structure.
  • Traversal: It means printing or accessing any or all elements in the data structure.
  • Deletion: It is a removal of an element from the data structure.
  • Searching: It means searching for a specific element in the data structure.
  • Sorting: It means organising the elements of the data structure in a particular sequence.

2. Define a stack. List the applications of a stack.

Stack is a linear type of data structure in which the elements get accessed using the Last in First Out (LIFO) approach. The basic operations that are performed on a stack include Push and Pop.

Image Source: Medium

Applications of a Stack

  • String Reversal
  • Conversion of Infix to Postfix
  • Solving a Postfix Expression
  • Checking the Balanced Parentheses
  • Memory Management
  • Conversion of an Expression to Another

3. Define queue. List some of its applications.

A queue is a linear type of data structure in which the elements get accessed using the First in First Out (FIFO) approach. A queue has the basic operations, including enqueue, dequeue, IsEmpty, IsFull, and Peek Applications of Queue.

Image Source: Medium

Applications of Queue

  • Scheduling the CPU Task
  • Processing the Website Request
  • Management of Input Stream
  • Finding Shortest Distance between Nodes of a graph Using BFS Algorithm
  • Buffer in a CD player, MP3 Player, and Similar Applications

4. Explain the priority queue.

A priority queue is a type of data type in the data structure. It is similar to a Queue with the only difference that the elements are assigned some priority. The Elements that have higher Priority get processed first as compared to the ones with lesser Priority. Two queues are required for the implementation of a Priority Queue. One is used to store the data, and the other for storing the Priority.

5. Explain the prefix, postfix, and infix notations.

  • Prefix Notation: The operator is written for the operands. For example, adding two numbers, A and B, can be written as + AB.
  • Postfix Notation: The operator is written after the operands in Postfix notation. For example, adding two numbers, A and B, can be written as AB+.
  • Infix Notation: In Infix Notation, the operator is written between the operands. For example, adding two numbers, A and B, can be written A+B.

6. How can you print the left view of a binary tree?

The Left View of a binary tree consists of the visible nodes when the tree is seen from the left side. To print the Left View of a tree, a tree gets traversed using a pre-order manner. The level information gets passed while traversing the tree.

If any level is reached for the first time while traversing the nodes, then the current node element and the current level are saved in the hash map. In this way, the hash map stores the first element of every level. Finally, the hash map gets traversed to get the answers.

Image Source: Imgur

The blue nodes in the image represent the left view of the binary tree – 5, 7, 14, 25.

7. How can you explain a graph data structure?

A graph is an example of a non-linear type of data structure. It stores the data elements as nodes or vertices, which get connected by the edges. Edges can be directed or undirected as per the data structure requirements.

8. Can you find the node’s height in a tree?

The height of a node is measured from the leaf node. It is the number of edges present in the longest path from the leaf node to the node. For example, the leaf node has a depth of 0.

9. What is the Selection Sort, and how does it work?

Selection sort is a simple sorting operation in which the elements are sorted in increasing order. That is done by picking the smallest number from the unsorted list and keeping it at the start. The process gets repeated until the elements get sorted in increasing order.

Below are the steps to perform selection sort:

  • Scan the unsorted elements and select the smallest number.
  • Switch the selected number with the one present at the first position.
  • Repeat the first two steps for the remaining elements.

The time complexity of the selection sort is O(n2) for both best and worst cases.

10. How push and pop operations are different?

Push and pop are the operations performed on a Stack. Below are differences between Push and Pop operations:

  • Push adds an element to the stack, and pop removes the items from the stack.
  • Push needs two arguments, i.e., stack name and element to add. The pop operation requires a single argument, i.e., stack name.
  • Stack gives a stack overflow error when Push operation gets performed on an already filled stack. Stack gives an underflow error when a pop operation is performed on an empty stack.

Frequently Asked Questions

What are Data Structure interview questions?

Data Structure interview questions are basic questions that test your understanding of Data Structure topics. These questions are mainly based on types, characteristics, operations, the different types of sorting methods, notations, linked list, queues, tree, graphs, and so on.

What are important topics in a Data Structure interview?

Linked list, graphs, number theory, searching, sorting, binary tree search are some important topics that you need to prepare for before attending a Data Structure interview.

How do you answer Data Structure questions?

Data Structure questions should be explained properly. If you can provide an example, then you should answer with the help of that example. If the question is more hands-on, then you should expand on the concept using paper and pen in the interview.

What are three characteristics of Data Structure?

Data structures have three characteristics:

1. Correctness: The interface implementation should be correct.
2. Time Complexity: Data structure operations should have minimum execution time.
3. Space Complexity: Data structure should use minimum memory.

What are the two main types of Data Structure?

The two main types of data structure are Linear, where elements are present in a linear sequence, and Non-Linear, where the data structure elements are arranged in a non-linear sequence, such as trees.

What are Data Structure and types?

A data structure is a method used to organise data so that it can be used more effectively. Data Structures are of two types: linear and non-linear.

Key Takeaways

These Data Structure interview questions will give you an idea about what you can expect in your interview. While you may only get a few of these Data Structure interview questions, it is still important to strengthen your understanding of the concepts. This will help you respond to any other question that has not been listed here. Learn more about data structure and Java from Coding Ninjas. Coding Ninjas offers several free courses that you can join to master concepts related to Java and data structure.

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