Top Coding Interview Questions in 2026

Coding Interview Questions for Beginners

Here we have discussed some popular coding interview questions, or interview coding questions, that will help you prepare the key concepts more efficiently and easily. 

1. What is an array?

An array is a data structure for storing a fixed number of elements that are all of the same type. It even allows for efficient organization of related data, which is able to provide quick access and searching operations.

2. How would you describe a linked list?

A linked list is a data structure where elements are chained together in a sequence. In this, each element is known as a node that holds data with a link to the next node. 

And there are different kinds of linked lists, like 

- Singly Linked Lists - In this, the nodes move only in one direction 
- Doubly Linked Lists - This type makes the node move in both forward and backward directions. 
- Circular Linked Lists - In this, the last node connects back to the first. 

3. What is a stack?

In structured data, a stack functions with specific operations in a last-in, first-out (LIFO) order. One more important thing to remember is that here the elements can only be retrieved beginning from the uppermost to the bottom element. 

This is among those questions that you must prepare efficiently for your list of coding interview questions. 

4. What do you know about the OOPs concept?

OOPs (Object-Oriented Programming System) is a paradigm that offers certain important concepts and topics related to objects and inheritance. 

It is very useful when it comes to organizing and structuring code for constituting certain concrete entities along with their elements. 

It is also beneficial in a way that it makes the code more scalable and smoother to operate. 

5. What are some of the important concepts in OOPs (Object-Oriented Programming Language)?

The important concepts that are introduced in OOPs  are:

I. Object—It is basically considered a concrete entity that has a specific behavior and state. It can also be defined as a specimen of a class.

II. Class: A class can be like a template that is used to define what an object will look like, along with its properties and actions.

III. Inheritance: This allows one class to use features (such as variables and methods) from another class. It also helps avoid repeating code by reusing what's already been written.

IV. Polymorphism: This means the same action can be done in different ways. In Java, this is done using method overloading and method overriding.

V. Abstraction: Abstraction hides the most complicated aspects of the code and only shows what is necessary. When it comes to Java, this is done using abstract classes and interfaces to focus on the what instead of how about the object.

VI. Encapsulation: This means keeping data and the code that works on it bundled together in one unit by making certain variables private and accessing them through methods. It is also helpful in a way that keeps the internal workings of a class safe as well as controlled. 

6. What do you understand by a binary search tree?

A binary search tree has the function of storing data and recovering it in a very smooth and efficient manner. The left side of a node holds values smaller than the node’s own value, while the right side holds values that are either equal to or greater than it. 

This is also one of the important coding interview questions that you can very much expect during the interview process. 

7. What do you understand by doubly linked lists? 

A doubly linked list is a very special kind of linked list through which we can move through the elements in both forward as well as backward directions. 

Along with this, one more important factor that we must remember is that the major reason why it is possible is because of the two pointers that are contained in each of the nodes, i.e., one that points to the next node and another that points to the previous one.

8. In what way can the variable declaration affect memory?

A variable declaration has a direct impact on how much memory is set aside for it. Along with this, another important thing is that the amount of memory that is allocated also depends on the type of data that the variable is required to hold. 

9. What do you understand by a Queue Data Structure or FIFO?

This concept of FIFO, or First In First Out, mainly comes from the strategies of how queues function. You can get an even easier understanding of this with the help of an example. You can think of a line of people waiting at a bus stop, and the person who is in line first is also the first one who will be served and leave. Therefore, this is actually a visual representation of how a queue data structure operates, and the first piece of data that is added is also the first one that will be removed. 

10. What is the best sorting algorithm out there?

There are a lot of different sorting algorithms that exist, like, 

- quick sort
- bubble sort
- balloon sort
- radix sort
- merge sort 

One key aspect that we must keep in mind is that there is no such algorithm that is referred to as the best, since it is mainly developed for a certain data structure where it performs its best function. 

Now, let's see some popular coding interview questions for the intermediate level. 

Intermediate-level Coding Interview Questions

1. How do you reverse an array in place?

Concept to use—two-pointer approach (start and end indices)
JS Code

function reverseArray(arr) {

    let start = 0, end = arr.length - 1;

    while (start < end) {

        [arr[start], arr[end]] = [arr[end], arr[start]];

        start++;

        end--;

    }

    return arr;

}

C Code:

void reverse(int* arr, int n) {

    for (int i = 0, j = n - 1; i < j; i++, j--) {

        int temp = arr[i];

        arr[i] = arr[j];

        arr[j] = temp;

    }

}

These types of popular coding interview questions, or interview coding questions, can be expected during the interview process. 

2. How do you find the K-th largest element in an array?

Concept to use: sort the array in descending order.

JS Code

function findKthLargest(nums, k) {

    nums.sort((a, b) => b - a);

    return nums[k - 1];

}

3. Write a code that can move all zeroes of an array to one end while the order of other elements is unchanged. 

Concept to use - Two-pointer overwrite method

JS Code

function moveZeroes(arr) {

    let nonZeroIndex = 0;

    for (let i = 0; i < arr.length; i++) {

        if (arr[i] !== 0) {

            arr[nonZeroIndex] = arr[i];

            if (nonZeroIndex !== i) arr[i] = 0;

            nonZeroIndex++;

        }

    }

    return arr;

}

4. How would you check for repeating elements in an array?

Concept: Sort and detect adjacent duplicates

C Code

void findRepeating(int arr[], int n) {

    Sort(arr, n);

    for (int i = 0; i < n; i++) {

        while (i < n - 1 && arr[i] == arr[i + 1]) {

            printf("%d ", arr[i]);

            while (i < n - 1 && arr[i] == arr[i + 1]) i++;

        }

    }

}

5. How would you reverse a linked list?

Concept to use—iterative reversal with three pointers

JS Code

function reverseLinkedList(head) {

    let prev = null, current = head;

    while (current !== null) {

        let nextNode = current.next;

        current.next = prev;

        prev = current;

        current = nextNode;

    }

    return prev;

}

These kinds of coding interview questions can also be expected for the interview process. 

6. In a linked list, how do you detect a cycle?

Concept to use—Floyd’s Cycle-Finding Algorithm

JS Code:

function hasCycle(head) {

    let slow = head, fast = head;

    while (fast !== null && fast.next !== null) {

        slow = slow. next;

        fast = fast.next.next;

        if (slow === fast) return true;

    }

    return false;

}

7. Merge two sorted linked lists into a single!

Concept to use - Use a dummy node and compare values

JS Code:

function mergeTwoSortedLists(l1, l2) {

    let dummy = new ListNode(0);

    let current = dummy;

    while (l1 && l2) {

        if (l1.val < l2.val) {

            current.next = l1;

            l1 = l1.next;

        } else {

            current.next = l2;

            l2 = l2.next;

        }

        current = current. next;

    }

    current.next = l1 || l2;

    return, dummy. next;

}

8. Write a program to find the factorial of a number (iterative and recursive).

C Code:     

// Iterative

for (int i = 1; i <= N; i++) ans *= i;

// Recursive

return N == 0 ? 1 : N * factorial(N - 1);

This is also one of the important coding interview questions that can be very much expected. 

9. Write a program to reverse a number (iterative and recursive).

C Code:

ans = ans * 10 + N % 10; // Reverse digit by digit

10. Check whether a number is a palindrome.

Concept: Reverse and compare

C Code:

if (rev == original) printf("Palindrome");

11. Write a program to find the GCD of two numbers.

Concept: Brute force check from min(a,b) to 1

C Code:

while (result > 0) {

    if (a % result == 0 && b % result == 0) break;

    result--;

}

12. Write a program in C that checks for two equal numbers without using == operator.

Concept: Use XOR

C Code:

if (!(x ^ y)) printf("Equal");

13. Write a program in C that will find the max and min of two numbers without using loops or conditionals.

C Code:

max = ((a + b) + abs(a - b)) / 2;

min = ((a + b) - abs(a - b)) / 2;

Now, let's go through some popular coding interview questions for the advanced level. 

Advanced-Level Coding Interview Questions

Now, let's go through some popular coding interview questions for the advanced level. 

14. How does Heap Sort work when sorting an array?

Heap Sort is a sorting technique that is mostly comparison-based, and it uses a special binary tree structure, which is called a heap. 

Here, we have provided a step-by-step process of how this actually works: 

Step-by-step (Using a Min-Heap for Ascending Order)

Firstly, it's very important to insert all the essential elements of the array into a min-heap

After this step is successfully completed, you must remove the smallest element from the heap and place it back into the array until the heap becomes empty. 

Some of the Java-style examples that use the built-in heap are : 

1. void heapsort(int[] arr) {

 PriorityQueue heap = new PriorityQueue<>();  // Min-heap

    for (int num: arr) heap.offer(num);                  // Add all elements

    for (int i = 0; i < arr.length; i++) arr[i] = heap.poll();  // Extract in order

}

In-Place Heap Sort (Without Extra Space)

If you are aiming for in-place sorting, the process can actually be a bit different:

- First, to make the largest element the root, convert the array into a max-heap. 
- Next, swap the root with the last element of the heap.
- Next, shrink the heap size by 1 to easily restore the heap property. 
- Finally, repeat the process until the heap is reduced to a single element.

Coding interview questions like this are also very important and can be expected during the interview process. 

15. What are the ways through which you find out whether a graph contains a cycle or not?

There are mainly two approaches that are in line with the graph type; these are mainly Undirected Graph and Directed Graph

Undirected Graph—You can easily start the DFS from any node and keep track of all the visited nodes along with their parent, and while doing DFS, if you are able to find a visited node that is not the parent, a cycle will exist. 

Directed Graph—When dealing with a directed graph, you can find a cycle by using two sets to monitor visited nodes and your current recursion stack. A cycle exists if you happen to find a node that's already in the recursion stack. 

16. Write an algorithm that can efficiently find if a given target value is present in a 2D matrix, where both the rows and columns are sorted in ascending order.

These are some steps that you can follow: 

1. You should start from the top-right corner
2. You should move towards the left when the present value is comparatively more than the expected target. 
3. After this, move down if it is smaller
4. Keep repeating this process until you either locate the target or step outside the matrix bounds, which means the matrix isn’t present.

17. What is your understanding of multi-threading

A thread is basically a lightweight unit of execution in a program. You can think of it as a single line of activity that starts and then follows a specific sequence of operations before it eventually ends. 

An important thing to remember is that, even though a thread functions within a program and makes use of certain resources, there are various new and emerging applications that benefit from making efficient use of multiple threads in a simultaneous manner. 

This even makes way for the various important tasks to take place in sync for that same application, which makes the functioning smoother and more efficient as well. 

Along with this, when it comes to material-intensive applications, making efficient use of multithreading can be quite beneficial to optimize the functioning and make sure there is a smooth and efficient user experience.

Yet, even after all these useful factors, the implementation of multithreading is not always easy since it needs a very thorough comprehension of the use of certain essential tools, along with a detailed knowledge and understanding of concurrency.

18. What do you mean by a regular expression? Also, explain the regular expression that is required for a phone number.

Through a regular expression, we are able to look for the general patterns, which are found mostly in text data.  

These also have a specific list of rules that are needed for explaining the pattern, no matter which programming language is used. 

Thus, we can also say that these regular expressions, also known as simply "annex," are very beneficial and are used popularly to function efficiently with the various text data.  

Therefore, if you prepare these coding interview questions thoroughly, it will become way easier for you to ace your interview process and start your career in your desired field.

Now, let's look at some tips to prepare efficiently for these coding interview questions.

Simple tips for coding interview questions

1. A very important tip that you must keep in mind while preparing for the interview is being consistent in your approach towards practicing.
You should be very patient as well as consistent while you are preparing the interview questions. 

2. Another simple but efficient tip that might help you in your preparation for the coding interview questions is designing a clear and strategic plan.
You can also make a proper schedule of the categories of different questions and prepare based on which questions and concepts to prioritize first. 

3. One more essential tip that might help you is developing clarity on the questions you practice. You shouldn’t jump straight to the code when you see the question.

Try to understand the problem first and then analyze the structure of the answer so that you can solve the code in a more efficient and smooth manner. 

Thus, these tips will surely be very helpful for you to prepare the coding interview questions in a more strategic and clever way.