Top 25+ DSA Interview Questions & Answers

Coding interviews contain a series of DSA questions. You should be skilled with arrays if you’re getting ready for your forthcoming tech interview with FAANG or another Tier-1 tech business.

In most coding interviews, it comes in second place to Strings. An array is a grouping of related data elements kept in close proximity to one another in memory.

As they are connected to all programming languages, such as C, C++, Java, Python, Perl, and Ruby, they are everywhere. Continue reading for some practice coding challenges and interview questions and answers based on arrays.

Python will be used in this post to tackle the coding issues because it is simple to use, comprehend, and must be familiar to most of us.

Let’s begin. 

1. How do you define an Array?

• A group of related data types is an array.
• Arrays are always fixed.
• The same kind of variable is stored in several places by array objects.
• Primitive types and object references are both compatible with it.

2. Dynamic Arrays: What Are They? What sets them apart from Basic Arrays?

The automatic scaling that dynamic arrays (also referred to as growable arrays, resizable arrays, changeable arrays, or ArrayLists in Java) provide is a significant advantage.

You must always know how many elements your array will store in advance since arrays have a fixed size. A dynamic array, on the other hand, grows as you add additional members to it, so you don’t need to know its exact size beforehand.

3. How do an array and a dictionary vary from one another?

This is a fundamentals-based array of interview questions that are regularly asked. The following are the key distinctions between arrays and dictionaries:

• An array is an ordered list of similar items. Dictionary, on the other hand, has key-value pairs.
• Array sizes can change dynamically. Such dynamic ideas do not exist in dictionaries.
• Before utilizing an array, its size must be specified. Dictionary sizes don’t need to be customized.
• Use the Redim statement if you wish to expand the array’s size. In dictionaries, an element can be added without a declaration.

4. List some of the benefits and drawbacks of arrays.

Advantages:

• Arrays can sort a number of elements simultaneously.
• Other data structures, like as stacks, queues, linked lists, trees, graphs, etc., can be implemented in an array.
• An index can be used to reach an element of an array.

Disadvantages:

• An array’s size must be declared in advance. At the moment of array declaration, we might not, however, be aware of the size we require.
• The structure of the array is static. It implies that array size is always fixed and that memory allocation cannot be increased or decreased.

5. What does “Sparse Array” refer to?

A sparse array is a data array that has a lot of entries with zero values. In contrast, a dense array contains the majority of its items with non-zero values. The indices of a sparse array, which converts numbers to objects, may include gaps. Compared to a HashMap, they are more memory-efficient.

6. When would you choose a linked list over an array?

When using linked lists instead of arrays, consider:

• You don’t need any elements to have random access.
• Where temporal predictability is essential, you need constant-time insertions and removals from the list.
• In order to create a priority queue, you might need to place items in the center of the list.
• You have no idea how long the list will be. If the array size rises, you must re-declare and duplicate memory, just like with simple arrays.

7. What distinguishes an indexed array from an associative array?

The primary distinctions between associative and indexed arrays are listed in the following table.

• A key-value pair in text or numeric format is used to sort an associative array. The indexed array’s keys are all numeric, and each key is connected to a distinct value.
• In an associative array, the key might be a string. Indexed array with integer keys starting at 0.
• A two-column table mimics the behavior of an associative array. Similar to a single-column table are indexed arrays.
• Maps are an associative array type. An index array is not a map.

8. What advantages does Heap have over sorted arrays?

The time efficiency of utilizing Heap over Sorted Arrays is the key benefit. While heap operations are quicker, sorting an array requires a lot of time. A heap can discover the smallest element considerably more quickly than an array can be sorted.

A given collection of numbers can be arranged in one of two ways using Sorted Arrays. On the other hand, for a given collection of numbers, there may be more than one potential heap.

9. Can we define the size of the array to be negative?

No, we cannot define a negative integer to be the size of an array. There won’t be a compile-time error if we declare. At runtime, we will, however, encounter a NegativeArraySizeException.

10. How do you locate the missing integer in a 1 to 100-element array?

The total of the series can be computed by applying the following function: n (n + 1) / 2

Only if the array doesn’t have any duplicates or has more than one integer missing will this function operate. Whether an array has duplicate elements, you can sort the array to see if there are any elements that are equivalent.

11. How do you find the index of an element in an array?

An element’s index can be discovered via a linear or binary search. Until it locates the match of the required element, a linear search function loops over each and every element in an array. It returns the index once it locates the matching element. Consequently, the linear search’s temporal complexity is O. (n). Both a sorted and an unsorted array can use linear search.

Using a binary search, which continually divides the array in half until the median of the interval matches the required element and provides the index, you can get the element’s index if the array is sorted. Consequently, the binary search’s temporal complexity is O. (log n).

12. How can you get rid of a specific element from an array?

Since you can’t simply delete elements from the original array since they are fixed sets with a defined size, the interviewer is seeking for you to suggest a different approach and deal with the problem that the question raises. The best course of action is to make a new array in order to delete an element. You may duplicate the elements from the first array in this array and only include the element you wish to delete.

Another strategy involves finding the target element in the array and then reversing the order of all the items that are to the right of the target element.

13. How can two arrays’ equality be verified?

You must first verify the lengths of the two provided arrays. The matching items of both arrays are compared when their lengths are equal. The two arrays will be regarded as equal. if each pair of components in every correspondence is equal. This approach is not advised to check the equality of two arrays if the arrays are large in size since it will take a lot of time. You may also use the equals() method included in the Arrays class, however, if the interviewer asks you to compare two arrays without utilizing built-in methods, this way will be useful.

14. When we discuss arrays, what do you mean by the phrases “Dimension” and “Subscript”?

The “Dimension” of an array is the number of indices, or subscripts, required to identify each individual member. Subscripts and dimensions might be unclear. A dimension is a description of the range of permitted keys, whereas a subscript is a number. There is just one subscript required for each array dimension.

For instance, the array arr[10][5] has two dimensions. Sizes 10 on one and 5 on the other. To address its components, you require two subscripts. Both are between 0 and 4; one between 0 and 9, inclusive.

Coding Interview Questions

15. Seek out a pair in an array that has the specified sum

For example,

Input:

• nums = [8, 7, 2, 5, 3, 1]
• target = 10

Output:

• Pair found (8, 2)
• Or
• Pair found (7, 3)

Input:

• nums = [5, 2, 6, 8, 1, 9]
• target = 12

Output: 

• Pair not found

16. Binary array sorting with linear time

Sort a binary array in linear time and in a fixed area. The output ought to display all zeros first, then all ones.

For example,

• Input:  { 1, 0, 1, 0, 1, 0, 0, 1 }
• Output: { 0, 0, 0, 0, 1, 1, 1, 1 }

A straightforward approach would be to calculate the array’s total number of 0s, say k, and then fill the first k indices in the array with 0s and the remaining indices with 1. As an alternative, we might calculate how many 1s are total in the array k, fill the last k indices in the array with 1, and leave the rest of the indices filled with 0.

The given approach has an O(n) time complexity and uses no additional storage, where n is the size of the input.

17. Find the largest two-int product in an array.

Find the largest product of two numbers in an integer array.

Think about the array 10 3 5 6 2 as an example. The (-10, -3) or (5, 6) pair is the highest product.

To think about every element combination and figure out their product is a foolish approach. If the product of the current pair is bigger than the maximum product obtained thus far, update the maximum product. Print the components of the final product last.

The above solution, where n is the amount of the input, has a time complexity of O(n2) and doesn’t take up any more space.

18. How to shift all of the array’s zeros to the end

Move all of the zeros in an integer array to the end. The answer should avoid using constant space and preserve the relative order of the array’s components.

Input: {1,2,3,0,8,0,4,7}

Output will be {1,2,3,8,4,7,0,0}

Put the element at the following available position in the array if the current element is not zero. Fill all remaining indices with 0 once the array’s items have all been processed.

The preceding solution has an O(n) time complexity, where n is the size of the input.

19. How to sort an array with two entries that are switched in one operation.

Sort an array in the linear time given two swapped items and an array with all of its elements arranged in ascending order. Pretend that the array contains no duplicates.

Input:= [1,9,3,4,7,2] or [9,3,7,2,1,4] or [2,4,1,7,3,9]

Output: = [1,2,3,4,7,9]

Beginning with the second element in the array, the objective is to compare each element to its predecessor. The position of the dispute is stored by taking two pointers, x, and y.

Update x to the previous element’s index and y to the current element’s index if the former is larger than the latter. Update y to the index of the current element if it turns out that the previous element is greater than the current element.

Finally, switch the elements at indexes x and y once we have finished processing each adjacent pair of elements.

Due to the fact that the aforementioned method only performs a single scan of the input array of size n, its time complexity is O(n). No additional room is necessary for the solution.

20. How to combine two sorted arrays in place.

Merge the items of arrays X[] and Y[]—two sorted arrays of size m and n each—by retaining the sorted order, that is, by filling X[] with the first m smallest elements and filling Y[] with the remaining elements.

If an element in the array X[] is already at the right position (i.e., the one that is the smallest among the remaining elements), disregard it; otherwise, replace it with the smallest element, which also happens to be the first member of Y[]. To retain the sorted order after swapping, transfer the element (now at Y[0]) to its proper location in Y[].

The first array’s size is m and the second array’s size is n, and the time complexity is O(m.n).

21. How to reorder an array of items in alternating high and low positions?

Rearrange an integer array so that each subsequent member is larger than the preceding and following elements. Assume the array doesn’t include any duplicate elements.

Sorting the array or utilizing additional space is not necessary for an effective approach. The plan is, to begin with, the second member of the array and go up by two for each loop iteration.

Swap the components if the last element exceeds the first one. In a similar vein, switch both items if the following element is bigger than the current element. We will obtain the desired array that complies with the specified restrictions at the conclusion of the loop.

22. How to substitute each element of an array without using a division operator with the product of each element in the array?

Without using the division operator, replace each element in an integer array with the product of all other elements.

In linear time and constant space, we can utilize recursion to address this issue. Recursively calculating the products of each element in the right subarray and passing the left subarray product as function parameters is the notion.

The time complexity is O(n).

23. Find the oddest element in an array in logarithmic time

Given an integer array in which all but one member have even numbers of occurrences, the problem is to determine how many times this one element appears. Find the odd occurring element in logarithmic time and constant space if the same elements occur in pairs in the array and there can never be more than two instances of a given element in a row.

The XOR operation enables us to resolve this issue in linear time. The goal is to XOR every element in the array. Only the odd occurring elements remain after the even occurring elements cancel each other out.

This problem can even be resolved in O(log(n)) time.

24. How to get the subsequent bigger element for each element in a circular array?

The next bigger element for each element in a circular integer array should be located. The first larger integer after an element x in the array is the subsequent greater element of that element.

From right to left, we may operate on array items. The goal is to loop for each element x until either the stack is empty or we have a higher element on top of it. Set the next bigger element of x to appear on top of the stack when it does.

25. Find an array’s inversion count?

Find the total number of inversions of an array. A pair I j) is referred to be an inversion of an array A if I j) and (A[i] > A[j]). We must count every pair of these in the array.

Counting all array members that are fewer than it to its right and adding the result to the output is a straightforward approach. 

This solution has an O(n2) complexity, where n is the size of the input.

26. What Is the Rain Water Trapping Problem?

Finding the most water that can be trapped in a given set of bars with a width of one unit each is known as the “trapping rainfall” issue.

The goal is to determine the highest bar that may be placed to the left and right of each bar. The minimum of the leading bars to the left and right, less the height of the current bar, is the quantity of water that is stored on top of each bar.

Conclusion

Compared to other data structure topics, arrays are simpler. In order to ace array interview questions, you need to have a fundamental understanding of arrays.

You should extensively review the foundations of arrays, including array operations (from declaring/creating an array to accessing/modifying array items), as well as programming concepts like loops, recursion, and basic operators in order to successfully answer array interview questions. Recognize the issue completely.

You should seek clarification if you have any queries. Think about dividing the issue into more manageable parts. Make sure you have the algorithm in mind before you begin programming; write it down or visualize it in a flowchart. then begin writing code.