Top 20 DSA Interview Questions and Answers for Freshers in 2024

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DSA, which stands for Data Structures and Algorithms, is a super important topic for job interviews, whether you're just starting or have some experience. It's a must-know for landing good jobs in tech. We get it—it can be challenging to figure out which DSA questions are the most important with so many out there. That's why we're here with the Top 20 most commonly asked DSA interview questions. These questions will help you ace your technical interviews without a hitch! 

Before proceeding further, we should know the basics of data structures and algorithms. Afterwards, we will understand the data structure interview questions. 

Understanding DSA in Brief

Data structures are the Lego bricks of computer programs. They're super important because they help us organize and work with data smartly. 

Data structures, like arrays, linked lists, stacks, and queues, are examples of how we organize data. They also help us write neat and organized code, making sure each part of the code does what it's supposed to do. Data structures ensure our programs run smoothly and do what we want them to do.

Basic DSA Interview Questions And Answers For Freshers

The following section will discuss the data structures and algorithms interview questions. 

Q1. What is Data Structure?

Ans. Data structures are the filing system of a program. Moreover, they help organize data in a way that makes sense to the computer. How we collect data affects how well the program works. There are different types of data structures, each used for multiple purposes. When we write code, we have to think carefully about how we structure data. If we don't do it efficiently or correctly, the code won't run as smoothly as we want it to.

Q2. Why is there a need to create Data Structures?

Ans. Creating data structures is necessary for a few reasons. First, they ensure each part of the code does its job well and quickly. They also help programmers spot and fix any issues in their code. Plus, they keep the code neat, making it easier to understand and work with.

Q3. What are a few applications of Data structures?

Ans. There are some applications of Data Structures. Some of them are as follows:

• Decision Making
• Image Processing
• Genetics
• Statistical and Numerical Analysis
• Blockchain
• Database Design 
• Compiler Design and many more

Q4. What is the process behind saving a variable in memory?

Ans. When we store a variable in memory, we first figure out how much memory it needs. Then, we assign that amount of memory to it. It depends on the type of data structure we're using. The variable is stored accordingly. We also use tricks like dynamic allocation to ensure everything is efficient and we can access the stored data whenever we need it. 

Q5. Describe the difference between storage structure and file structure.

Ans. File Structure: It organizes your files on a hard drive or USB stick. The data stays there until you delete it.

Storage Structure: Here, data is kept in the computer's memory (RAM) and is erased once it's done being used.

The main difference is that the storage structure keeps data in the computer's memory, while the file structure keeps it on external devices like hard drives.

Here are five main data structures and algorithms interview preparation. Further, we will elaborate on more.

Q6. What are the different types of Data Structures?

Ans. There are two types of Data Structures. They are as follows: 

• Linear Data Structure: Suppose a line of people standing in a queue, where each person is connected to the person in front and behind them. That's how data is arranged in a linear data structure, like in arrays or linked lists.

• Non-Linear Data Structure: Unlike a straight line, non-linear data structures are more like a web or a tree, where data elements are connected in various ways. You can't just walk through all the elements in one go like you can in a straight line. Trees and graphs are examples of non-linear data structures.

Q7. Define Stack Data Structure. What is its application?

Ans. A stack is a pile of items where you can only add or remove things from the top. It follows a rule called LIFO (Last In, First Out) or FILO (First In, Last Out), which means the last thing you put on the stack is the first thing you take off. In short, it's all about the order in which you add and remove things.

Following is a list of its applications:

•  A stack helps keep track of all the steps and their data until you finish.
• Redo and Undo operations in doc editors.
• To reverse a word or sentence, you can use a stack to flip the characters in the correct order.
• In programming, a stack helps ensure that parentheses and other brackets are correctly matched and closed.
• Converting mathematical expressions from one form to another often involves using a stack to rearrange the elements correctly.
• When a program calls functions, a stack keeps track of the order in which and helps manage to return to the right place after each function finishes.

Q8. Describe Multiple Operations in Stack Data Structure.  

Ans. Here are the various operations you can do with a stack:

• Push: Adding something to the top of the stack. If the stack is already complete, it's trying to add too many things to a full backpack.
• Pop: Taking something off the top of the stack. If the stack is empty and you try to take something off, it's reaching into an empty bag.
• Top: Checking what's on the top of the stack without removing it.
• Isempty: Checking if the stack is empty.
• Size: Finding out how many things are in the stack. 

Q9. Explain Queue Data Structure. Describe its application.

Ans. A queue is a line where people wait for their turn. You add new people to the back of the line, and they leave from the front when it's their turn. It's like how you wait at a store—the first in line gets served. Queues are handy when you need to keep things organized in order and serve them one by one. 

Here are a few applications of queue data structure. 

• Searching for something in a maze by checking all the paths simultaneously, starting from where you are.
• Managing tasks on your computer to make sure everything runs smoothly.
• Handling calls fairly and efficiently.

Q10. Explain various operations used in the queue data structure.

Ans. Here are some operations utilized in the queue data structure. 

• Enqueue: Adding something to the back of the line. If the line is already full, it's trying to add too many things to a bag.
• Dequeue: Taking something from the front of the line. If the line is empty and you try to take something off, it's reaching into an empty bag.
• IsEmpty: Checking if the line is empty. 
• Rear: Checking what's at the back of the line without removing it.
• Front: Checking what's at the front of the line without removing it.
• Size: Counting how many things are in the line. 

Till above, we’ve prepared you with some critical questions with DSA interview preparation. Further, we have additional too. 

Q11. How to execute a queue using stack?

Ans. Follow the below steps to execute a queue using stack:

• By making enqueue operation costly: 
  • To add something to the queue (enqueue), we need to move all existing items from stack1 to stack2, add the new item to stack1, and then move everything back to stack1.
  • To remove something from the queue (dequeue), we simply take the top item from stack 1. 
  • This method makes adding items to the queue slower (O(n) time complexity) but removing items is fast (O(1) time complexity).

• By making the dequeue operation costly:
  • To add something to the queue (enqueue), we just put it on top of stack1. 
  • To remove something from the queue (dequeue), if stack2 is empty, we move all items from stack1 to stack2 (reversing the order), then take the top item from stack2. 
  • This method makes adding items to the queue fast (O(1) time complexity) but removing items slower (O(n) time complexity). 

Q12. How to Execute Stack Using Queues?

Ans. Follow the below steps to execute the stack using Queues. 

• By making push operation costly:
  • To add something to the stack (push), we move all existing items from q1 to q2, add the new item to q1, and then move everything back to q1.
  • To remove something from the stack (pop), we simply take the top item from q1.
  • This method makes adding items to the stack slower (O(n) time complexity) but removing items is fast (O(1) time complexity).

• By making pop operation costly:
  • To add something to the stack (push), we just put it in q1.
  • To remove something from the stack (pop), if q2 is empty, we move all items from q1 to q2 except the last one (reversing the order), then take the last item from q1.
  • This method makes adding items to the stack fast (O(1) time complexity) but removing items slower (O(n) time complexity).

Q13. Explain Array Data Structure and its application.

Ans. An array is an extensive container where we can store much data in an organized way. It makes a list of items, but with an array, we can store a lot more things. We create an array by putting smaller arrays together. Each part of the array has its name, and the data is kept in the order we put it in. 

Arrays are often used in databases and computers to hold a ton of data smartly. They're great for storing things we use enough, like lots of text or pictures, because they help us find and use that information quickly.

Q14. Define types of Array Data Structure

Ans. Here are the different types of arrays:

• One-dimensional array: It is a long line where we keep things in order. We can find things by just knowing their position in the line.

• Two-dimensional array: Suppose, a table with rows and columns, like a spreadsheet. We use rows and columns to organize data, making it easy to find things based on their row and columns.
• Three-dimensional array: It’s a cube with layers, rows, and columns. We use three sets of numbers to find things: layer number, row number, and column number. In short, it finds a specific spot in an ample cube. 

Q15. Explain the linked list data structure and its application.

Ans. A linked list is a chain of connected items. Each item is linked to the next one in line. Imagine a train where each carriage is connected to the next. For example, the order of the carriages in the train is the same as the order they were added. 

Some applications of the linked list data structure are as follows. 

• Linked lists help make these structures work by connecting items in a chain-like manner.
• The operating system can adjust and allocate memory as needed.
• Round-robin scheduling makes sure each task gets some time to run. Moreover, it takes turns like a fair game. 
• When you click forward or back in your browser, it moves through a linked list of web pages, where each page is connected to the next and previous ones.

Here, we have mentioned the other DSA questions and answers. But it’s not the end. There are some other questions, too. 

Q16. Briefly explain the types of linked list data structures. 

Ans. Linked List Data Structure has several types. They are as follows:

• Singly Linked List: It is a chain where each link points to the following link. As a result, it is valid for lists like shopping lists or records where items are added one after another.
• Doubly Linked List: This is like a chain where each link not only points to the next one but also to the previous one. It's faster for accessing data but can be harder to manage.
• Circular Linked List: It’s a circular path where the end connects back to the beginning. The last item points to the first in this list, making it circular.
• Doubly Circular Linked List: Similar to a doubly linked list but circular, so the last item connects back to the first, and the first's previous points to the last.
• Header List: This is a list with an extra "header" node at the start, which helps with specific calculations and operations on the list.

Q17. Define asymptotic analysis of an algorithm.

Ans. Asymptotic analysis of an algorithm looks at how fast or slow an algorithm runs based on its mathematical rules. Moreover, it helps us describe how well an algorithm performs in different situations - the best-case, average-case, and worst-case scenarios.

Q18. Explain hashmap in a data structure.

Ans. A Hashmap is a smart storage system that quickly finds what you're looking for. It uses a special method called a hash table. Additionally, it helps you access data fast, almost instantly, if you know the key (like a secret code). 

Q19. What does an object need to have to be used as a key or value in a HashMap?

Ans. To use an object as a key or value in a HashMap, it needs to have two solutions:

• An equals() method: This helps compare if two objects are equal.
• A hashcode() method: This gives each object a unique code, like an ID, which helps the HashMap store and retrieve data quickly.

Q20. How does HashMap manage collisions in the Java Language?

Ans. HashMap manages collisions in the Java Language through -  

• The HashMap class in Java uses a method of chaining to handle situations where multiple values with the same key are added. When this happens, the values are stored in a linked list inside a bucket corresponding to the key.
• However, in the worst-case scenario, all keys may end up with the same hashcode, causing the hash table to change into a long linked list. However, it can slow down the search process significantly, taking more time (O(n) complexity) instead of the usual quick time (O(1) complexity). So, it's necessary to pick the proper hashing algorithm to avoid this issue.

Conclusion

These questions and answers are essential for freshers preparing for data structures and algorithms interviews in 2024. Knowing these concepts well will help you get ready for DSA interview questions and perform better. We hope we’ve cleared all your doubts. So, just prepare it for the future interview. 

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