# Data Structure Interview Questions and Answers

Posted in /  Vinay Khatri
Last updated on September 28, 2023

Every year many computer science graduates apply for jobs related to programming, coding, and software development in big tech companies such as Google, Microsoft, Amazon, Netflix, and so on. However, many of them usually don't have an idea of the questions that they might have to face in the interview.

In most coding interviews, the interviewers ask several questions related to data structure and algorithms because these are fundamental topics that computer science graduates should know.

This article covers the most frequently asked data structure interview questions and their appropriate answers. So, if you are appearing for a job interview that involves coding or programming , becoming familiar with these questions can help you give your best. So, let's get started!

## Best Data Structure Interview Questions

We have categorized the following most frequently asked data structure and algorithm interview questions into three levels: beginner-level data structure interview questions, intermediate-level data structure interview questions, and advanced-level data structure interview questions.

### Beginner-Level Data Structure Interview Questions

#### 1. What is a data structure?

Answer: Data structure refers to a format for storing, organizing, and manipulating data efficiently. Arrays, LinkedLists, Trees, and Graphs are examples of data structures. As there are different forms of data structures, with each one having a specific way of organizing and relating data that they have stored, we can use data structures according to the requirements of a project.

#### 2. What is the key difference between file structure and storage structure?

Answer: In the storage structure, RAM is allocated to the variables of a function, and as soon as the function ends, the memory allocated to the variables also gets deleted. In file structure, the file created by the program is stored in the secondary memory and remains intact until it gets deleted explicitly.

#### 3. Name all the basic operations we can apply in data structures?

• Deletion – To delete an element from the data structure.
• Insertion - For inserting a new element in the data structure.
• Searching - To search an element in the data structure.
• Sort - For arranging the elements of a data structure in ascending or descending order.
• Traversal - Go through each element of the data structure.

#### 4. When can we use a binary search on data structure?

Answer: Binary search is a searching algorithm with a time complexity O(logn), and it can be applied to a sorted array or list. This algorithm search divides an array into two equal parts and searches for the middle element.

#### 5. What is the key difference between the Breadth-First Search and Depth First Search algorithms?

Answer: Breadth-First Search (BFS) uses the queue data structure as an auxiliary data structure, whereas the Depth First Search algorithm uses a stack data structure .

#### 6. What is a linked list?

Answer: A linked list is a linear data structure and a collection of nodes. In the linked list, each node is connected to the successor node except the last one.

#### 7. Explain a stack and enumerate some of its applications.

Answer: A stack is a linear data structure that follows the principle of Last In First Out (LIFO), and according to LIFO, the element which is inserted at last will be retrieved first. In a stack, we represent the insertion operation with push and deletion operation with pop. Following are some notable applications of a stack:

• Check the balanced parentheses in an expression.
• To estimate the postfix expression.
• Reverse a string.

#### 8. How can we traverse an array?

Answer: By using a loop and the index value, we can go through every element of an array, but the loop should start from 0 and end with one less than the total size of the array.

#### 9. What is a queue, and how is it different from the stack?

Answer: A queue is a linear data structure that works on the principle of First In First Out (FIFO). According to FIFO, the element inserted first in the queue data structure would be retrieved first. On a queue, we can perform operations like dequeue and enqueue. Following are some key differences between a queue and a stack:

• A queue follows FIFO, whereas a stack follows the LIFO principle.
• If we perform the stack push and pop operation on a string, we get a reverse string, but in the queue, we do not get any reverse string.

#### 10. What are arrays?

• An array is a collection of homogeneous data elements, which means that we can only store similar data types in the array data structure.
• When we declare an array, we also mention its size so the compiler can allocate contiguous memory locations to its elements.
• As all the elements of the array are stored in the consecutive memory location, we can access its element using index values.
• The index value of the array starts with 0 and ends with 1 less than the total size of the array.

#### 11. Where do we apply the data structure?

Answer: The data structure is applied everywhere, and even every operating system needs a data structure to perform basic operations on data. Data structures provide an efficient way to design operating systems, AI machines, compilers , databases, graphics, statistical tools, and so on.

#### 12. What do you understand by LIFO and FIFO?

• LIFO : LIFO stands for Last In First Out, and it is a technique to implement a stack data structure where the element added last in the structure would be accessed first.
• FIFO: FIFO stands for First In First Out, and it is the opposite of LIFO. In FIFO, the element inserted first in the structure would be accessed first.

#### 13. What are binary trees?

Answer: A binary tree is a tree data structure where each node has at most two child nodes, which are also referred to as right and left nodes.

#### 14. What is dynamic memory allocation, and how does it helps in managing data?

Answer: In dynamic memory allocation, memory is allocated to the program variables and functions at the run time. It combines separately allocated structure blocks to form a composite structure that can expand and contract according to the need. In simple words, in dynamic memory allocation, data structures are allowed to store and remove data of arbitrary size in arbitrary order.

#### 15. Which data structure is recursion used to perform its task?

Answer: Recursion is a technique in which a function calls itself again and again until a base condition becomes true, and at the backend, the recursion technique uses the stack data structure to perform its task.

#### 16. What is a Binary Search Tree (BST)?

Answer: A binary search tree is an ordered binary tree in which the left node key value is less than the parent node key value, and the right node key value is greater than or equal to the parent node key value.

#### 17. What is the key difference between NULL and Void?

• NULL: NULL is a value that represents empty or nothing.
• Void: Void is a data type identifier, and we cannot use it as a value.

#### 18. What are multidimensional arrays?

Answer: A multidimensional array uses multiple indexes to store data. It can be represented as an array inside an array. Many real-world problems cannot be solved with the 1-D array, so we use the multidimensional array to tackle those problems.

#### 19. What is the difference between PUSH and POP?

Answer: PUSH and POP are operations that can be applied to the stack data structure. While PUSH adds an element at the top of the stack, POP retrieves the last PUSHED element of the stack.

#### 20. What is an ordered list?

Answer: In an ordered list, each node of the list contains a key value, and using this key value, we can traverse through each node of the list. Also, the key-value pairs form an increasing order as the list is traversed.

#### 21. How does the variable declaration affect memory allocation?

Answer: When we declare a variable with its data type, the compiler allocates memory to the variable according to its data type. For example, if we declare an integer data type variable, the compiler will allocate 4 bytes of memory to that variable.

#### 22. What are linear and non-linear data structures?

Answer: Linear Data Structure: A linear data structure arranges all of its elements in a sequential manner, but it is not necessary that elements are stored in the consecutive memory location. Arrays, Linked Lists, Stacks, and Queues are examples of linear data structures. The time complexity of traversal operation in a linear data structure always remains O(n). Non-Linear Data Structure: Non-linear data structure does not arrange its elements in sequential order but uses different levels to store the elements. The time complexity of traversal operation in the non-linear data structure is always greater than O(n).

#### 23. What is merge sort?

Answer: Merge sort is a sorting algorithm that can arrange the data structure elements in ascending or descending order, and it works on the principle of Divide-and-Conquer. Working: In merge sort, we first divide an array or a list into two halves and further continue to divide those equal halves into more equal halves until we get an atomic value. Once we have the atomic values, we combine those values in exactly the same manner as we broke them, but before that, we compare them.

#### 24. Explain data abstraction and how it is different from data encapsulation?

Answer: Data abstraction is a property of object-oriented programming in which we try to hide the essential information from the user by wrapping the complete program information in a single unit. We implement data abstraction using classes and objects. Data abstraction is an integration of data encapsulation, and data encapsulation deals with the concept of wrapping various data members and function members in a single unit. On the other hand, data abstraction deals with hiding data. It’s the data encapsulation that leads to data abstraction.

### Intermediate-Level Data Structure Interview Questions

#### 25. What are the key advantages of a linked list over an array?

• A linked list can be used to hold different data types at once.
• A linked list is more flexible. We can insert and delete elements quite easily.
• It is scalable and memory efficient.

#### 26. How can you insert a new element in the binary search tree?

Answer: Here are the steps to insert a new element in the binary search tree:

• First, we need to find the correct location where the new element will be linked.
• We will then traverse from the root node.
• If the data is less than the key value, we need to search for the empty location in the left subtree and insert data.
• If the data is greater than the key value, we need to search for the empty location in the right subtree and insert data.

#### 27. What is a linear search?

Answer: A linear search is a searching algorithm with the worst time complexity of O(log n), where n is the total number of elements present in the list or array. In this algorithm, we check for each element and compare it with the target element.

#### 28. How does selection sort work on an array?

Answer: Here are the steps to perform a selection sort on an array:

• Initially, we start with the first element of the array, search for the smallest element in the array and replace it with the first position element.
• Next, we will move to the second position and again search for the smallest element from the second position to the last position of the array, and once we find the smallest element, we will swap it with the second element. Now, two elements in the array have been sorted.
• Lastly, we will move to the third element and continue to repeat the above steps until the array gets sorted.

#### 29. What is the advantage of using a heap over a stack?

• It is more flexible.
• Memory space for the heap can be dynamically allocated and de-allocated as needed.
• Heap does not have a limit on the memory size.

#### 30. Explain recursion and the measures we should take care of when we create a recursive function.

Answer: When a function calls itself again and again, then that scenario is known as recursion, and it is the most common and best technique used to traverse through the non-linear data structure. To design a recursive function, we should take care of two measures, namely the base condition and the progressive approach.

• Base Condition: Each recursive function must have a base condition where it returns a single value or where it stops calling itself.
• Progressive Approach: A progressive approach is a statement that brings the recursive call near to the base condition.

#### 31. How signed and unsigned numbers affect memory?

Answer: The signed number represents positive and negative integers, whereas the unsigned number represents a positive number, including 0. Also, both signed and unsigned numbers take 8 bits of memory. In a signed number, 1 out of 8 bits is used to represent the positive and negative sign, which means that only 7 bits of the signed number are capable of holding numeric values. This is also the reason why the signed number has a range of -128 to 127. On the other hand, unsigned numbers use 8 bits to store the numeric value, so its range varies from 0 to 255.

#### 32. What is a postfix expression?

Answer: In postfix expression, the operator comes after its corresponding operands. For instance, in infix and mathematical expressions, if we want to apply an additional operation between two operands, we put the operator between the operands. However, in postfix, we put the operator after the operands. In infix ----->A + B Postfix expression -----> AB+

#### 33. What are the minimum numbers of nodes a binary tree can have?

Answer: A binary tree can have a minimum of 0 nodes, which means it does not even have a root node.

#### 34. Does declaration mean fixed memory allocation for all the variables at compile time?

Answer: Except for the pointer variable, when we declare a variable, the compiler allocates a temporary memory to the variables. However, the actual memory allocation for the data occurs at the runtime.

#### 35. Name the different data structures where we can apply pointers.

Answer: Pointers are normally used with nodes, and the data structure that consists of nodes, uses pointers. Data structures that can use pointers are:

• Stack
• Trees
• Graphs

#### 36. What is the AVL tree?

Answer: Adelson, Velski & Landis (AVL) tree is a self-balancing binary tree. If we insert a new element in the AVL tree, the tree will re-balance itself and keep its height as small as possible.

#### 37. What is the minimum number of queues required to implement a priority queue?

Answer: To create a priority queue, we require at least two queues, one for the sorting priorities and the second for the actual storage of data.

### Advanced-Level Data Structure Interview Questions

#### 38. Name some of the sorting algorithms with their corresponding time complexities.

 Algorithm Time Complexity Best Average Worst Selection Sort ?(n^2) ?(n^2) O(n^2) Bubble Sort ?(n) ?(n^2) O(n^2) Insertion Sort ?(n) ?(n^2) O(n^2) Heap Sort ?(n log(n)) ?(n log(n)) O(n log(n)) Quick Sort ?(n log(n)) ?(n log(n)) O(n^2) Merge Sort ?(n log(n)) ?(n log(n)) O(n log(n)) Bucket Sort ?(n+k) ?(n+k) O(n^2) Radix Sort ?(nk) ?(nk) O(nk)

Here, n is the number of elements present in the array, and k represents the maximum number length of the largest digit.

#### 39. What is dequeue?

Answer: Dequeue stands for a double-ended queue , and it is a queue data structure in which we can add and remove elements from either side of the queue.

#### 40. What is bubble sort, and how does it work?

Answer: Bubble sort is a sorting algorithm with the worst time complexity O(n 2 ). In bubble sort, we compare each element with its adjacent element and swap them if they are not in the right order. The bubble sort gets its name due to the fact that every time the comparison takes place between two elements, those elements act like bubbles.

#### 41. What is a notation in the data structure, and how many types of notation are there?

Answer: In the data structure, how we write an expression and how the operator is written along with the operands is decided by the notation. In general, we have three types of notations in the data structure.

1. Infix notation - In this notation, the operator is written between the operands, e.g., A + B.
2. postfix notation/reverse polish Notation - In this notation, the operator is written after the operands, e.g., AB+.
3. Prefix notation/polish Notation - In this notation, the operator is written before the operands, e.g., +AB.

#### 42. Name the parts of the linked list.

Answer: A linked list is a collection of nodes in which each node points to another node, except for the node at the end. The first node of the linked list is known as Head, and the last node is known as Tail.

#### 43. Explain the various types of linked lists present in data structures.

• Singly Linked List - In the singly linked list, each node points to its successor node except the last one. Here the last node points to the NULL value.
• Doubly Linked List - In this linked list, each node contains two pointers. Also, the first pointer points to the next node, and the other points to the previous node.
• Circular Linked List - Circular linked list is similar to the singly linked list but here, the last node points to the head node rather than pointing to the NULL value.

#### 44. What is a graph?

Answer: A graph is a non-linear data structure that comprises nodes and arcs. Here the node represents the vertices, and arcs represent the edges. Also, the arc act as a bridge between the two nodes or vertices.

#### 45. What is the Huffman algorithm?

Answer: Huffman’s algorithm is used for creating extended binary trees that have minimum weighted path lengths from the given weights. It makes use of a table that contains the frequency of occurrence for each data element.

#### 46. How to form a queue data structure using a stack and vice-versa?

Answer: Queue using Stack: We require two stacks to form a queue-like data structure. The first stack takes the element by push operation, and once the stack is full, we perform the pop operation on the first stack. Also, we use the push operation on the second stack and push all the popped elements of the first stack into the second stack. So, when we pop from the second stack, it will act as a queue. The first element which was pushed in the first stack will be popped first from the second stack. Stack using Queue: With the help of two queue data structures, we can form a stack-like data structure. The first queue is used to insert the element in the queue, and the second queue will store the popped elements of the first queue. That is how the second queue will pop the last entered element first, and the complete structure acts as a stack data structure.

#### 47. Define the different types of algorithm approaches.

• Divide and Conquer: In this approach, we divide a problem into smaller sub-problems. Next, we solve those small subproblems independently and then join them back.
• Dynamic Programming: Dynamic programming is similar to the divide and conquer approach, but in dynamic programming, we solve all the subproblems together.
• Greedy Approach: In the Greedy approach, we try to solve the problem by choosing the best option.

#### 48. What is the interpolation search technique?

Answer: The interpolation search technique is a searching algorithm that is similar to the binary search algorithm, which divides the array into two parts. However, interpolation does not divide the array into two equal halves. Rather, it tries to divide the array from that position from where it can get near to the solution. The interpolation search technique uses both Divide and Conquer and Greedy approaches.

#### 49. How can you check if a given Binary tree is a BST or not?

Answer: We perform the in-order traversal on the binary tree and store each visited element in an array. If we get a sorted array, it would be a Binary Search tree .

#### 50. What is a Fibonacci search?

Answer: A Fibonacci search is a search algorithm that applies to a sorted array. It makes use of the divide-and-conquer approach that can significantly reduce the time needed in order to reach the target element.

## Conclusion

The aforementioned questions are the most frequently asked data structure interview questions, and we recommend you to go through these questions to give your best in your upcoming interview. Also, these questions and answers will help you brush up on your knowledge of data structure so that it becomes easier for you to leave a lasting impression on the interviewer.

Good luck!

If you have any suggestions related to this article, or you want to share any commonly asked data structure interview questions that are not listed in this article, you can do that in the comments sections below.

## FAQs

First, have a good grasp of every concept of DSA. Try solving problems based on DSA on coding platforms, such as CodeChef, CodeLeet, etc. Make sure to revise every CS concept. Go in-depth about your projects. Finally, read the above list of DSA interview questions to recollect all concepts.

If you invest a lot of time in learning DSA and have the proper resources, you can learn DSA in 3 months. However, to master DSA, it will need more time, i.e., around 6 months.

C++ is the best and the most preferred language for practicing DSA.

Yes, the concepts of DSA are the same across every programming language. However, only the implementation varies.