Blackrock Backend Developer Interview Questions and Answers

Round duration - 60 minutes
Round difficulty - Easy

This was the qualification round which comprised of several MCQs and 3 coding questions.
It was held on Hackerearth from 3:00 PM to 4:00 PM.
Use of any other IDE was prohibited.

Q1. Given a problem statement and a piece of code, how would you find and correct the bug in the code?
Ans. 
To find and correct a bug in code, analyze problem statement, review code, use debugging tools, and test different scenarios.
• Understand the problem statement and expected output.

• Review the code for syntax errors, logical errors, and potential bugs.

• Use debugging tools like breakpoints, print statements, and IDE debuggers.

• Test the code with different inputs to identify the bug.

• Make necessary corrections based on the iden...read more
Answered by AI
Add your answer
Q2. 
Print Nodes at Distance K from a Given Node
Given an arbitrary binary tree, a node of the tree, and an integer 'K', find all nodes that are at a distance K from the specified node, and return a list of th...read more
Ans. 
Given a binary tree, a target node, and an integer K, find all nodes at distance K from the target node.
• Traverse the binary tree to find the target node.

• Use BFS to traverse the tree from the target node to nodes at distance K.

• Keep track of the distance while traversing the tree.

• Return the values of nodes at distance K in any order.
Answered by AI
Add your answer

Round duration - 90 minutes
Round difficulty - Medium

I was shared a link of Google Meet and the Google Docs was shared where there was 1 coding problem to be coded there and then the code was run on an IDE to check the sample tests.
Then the interview was followed by a lot of Operating System and Computer System Architecture Questions.
There were 2 Interviewers and both were helpful.
The timing was from 2:30 PM to 4:00 PM

Q1. 
Subarray Challenge: Largest Equal 0s and 1s
Determine the length of the largest subarray within a given array of 0s and 1s, such that the subarray contains an equal number of 0s and 1s.
Input:
Input beg...read more
Ans. 
Find the length of the largest subarray with equal number of 0s and 1s in a given array.
• Iterate through the array and maintain a count of 0s and 1s encountered so far.

• Store the count difference in a hashmap with the index as the key.

• If the same count difference is encountered again, the subarray between the two indices has equal 0s and 1s.

• Return the length of the longest subarray found.
Answered by AI
Add your answer

Round duration - 45 minutes
Round difficulty - Easy

The round was held on Google Meet with HR from 2:30 PM to 3:15 PM.
The HR was friendly and asked the basic questions.

Round duration - 90 minutes
Round difficulty - Easy

Consisted of 3 DSA questions and MCQ on python proficiency

Q1. 
Palindromic Substrings Problem Statement
Given a string S, your task is to return all distinct palindromic substrings of the given string in alphabetical order.
Explanation:
A string is considered a pal...read more
Ans. 
Return all distinct palindromic substrings of a given string in alphabetical order.
• Iterate through all possible substrings of the given string.

• Check if each substring is a palindrome by comparing it with its reverse.

• Store all palindromic substrings in a set to ensure uniqueness.

• Return the sorted list of palindromic substrings.
Answered by AI
Add your answer

Round duration - 60 minutes
Round difficulty - Medium

It was a pure DSA round and interviewer was very helpful with providing significant hints to solve the given problems

Q1. 
Island Perimeter Calculation Problem
Given a binary grid representation of a map of an island, calculate the perimeter of the island. The grid uses '0' for water and '1' for land.
The grid has only one i...read more
Ans. 
Calculate the perimeter of an island represented by a binary grid.
• Iterate through the grid and count the perimeter based on land cells and their adjacent cells.

• Each land cell contributes 4 units to the perimeter, subtract 2 units for each adjacent land cell.

• Handle edge cases where land cells are at the boundaries of the grid.

• Return the total perimeter for each test case.
Answered by AI
Add your answer

Round duration - 60 minutes
Round difficulty - Hard

DSA+OS

Q1. 
Clearing the Forest Problem Statement
Ninja lives in a city called Byteland where a festive event is being organized. To make space for this event, Ninja is tasked with clearing a nearby forest. The fores...read more
Ans. 
Calculate the minimum number of steps Ninja needs to cut down all trees in a forest grid.
• Iterate through the grid to find the shortest path to cut down all trees in order.

• Use a priority queue to keep track of the shortest trees to cut next.

• If it's impossible to cut all trees, return -1.

• Consider all four cardinal directions for movement in the grid.
Answered by AI
Add your answer

Round duration - 70 Minutes
Round difficulty - Hard

The test link was active for a week, and you can give anytime you like. The IDE was very good and self explanatory. There were 2 coding questions and 15 aptitude questions.

Q1. 
Unweighted Graph Shortest Path Problem
You are tasked with finding the shortest path between two houses in the city of Ninjaland, represented as an unweighted graph. The city has N houses numbered from 1 ...read more
Ans. 
Find the shortest path between two houses in a city represented as an unweighted graph.
• Use breadth-first search (BFS) algorithm to find the shortest path in an unweighted graph.

• Start BFS from the source house and keep track of the path taken to reach each house.

• Once the destination house is reached, backtrack from destination to source to find the shortest path.

• Consider using a queue data structure to implement BFS eff...read more
Answered by AI
Add your answer
Q2. 
Check If Two Nodes Are Cousins
You are given an arbitrary binary tree consisting of N nodes, where each node is associated with a certain value, and two node values, a and b. Your task is to determine if ...read more
Ans. 
Check if two nodes in a binary tree are cousins by comparing their levels and parents.
• Traverse the tree to find the levels and parents of the given nodes.

• Compare the levels and parents of the two nodes to determine if they are cousins.

• If the levels are the same and the parents are different, the nodes are cousins.
Answered by AI
Add your answer

Round duration - 60 Minutes
Round difficulty - Medium

It contained two programming questions.

Q1. 
Counting Nodes in a Complete Binary Tree - Problem Statement
Given the root of a complete binary tree, calculate the total number of nodes in this tree.
A complete binary tree is defined as a binary tree...read more
Ans. 
Count the total number of nodes in a complete binary tree given its root.
• Traverse the tree in level order and count the nodes

• Use a queue to keep track of nodes at each level

• Check for null nodes represented by -1 in the input

• The total number of nodes in the example tree is 7
Answered by AI
Add your answer
Q2. 
Rearrange Array Numbers for Largest Possible Number
Given an array ARR consisting of non-negative integers, rearrange the numbers to form the largest possible numerical value. You are not permitted to alt...read more
Ans. 
Rearrange array numbers to form the largest possible numerical value by combining digits of each number in the array.
• Convert integers in the array to strings for easier manipulation.

• Sort the array of strings in non-increasing order based on custom comparison function.

• Join the sorted strings to form the largest possible number.
Answered by AI
Add your answer

Round duration - 60 Minutes
Round difficulty - Medium

This was more of a interaction round with some technical skills.

Q1. Can you explain the ACID properties and the rollback mechanism in DBMS?
Ans. 
ACID properties ensure database transactions are processed reliably. Rollback mechanism undoes changes if transaction fails.
• ACID properties: Atomicity, Consistency, Isolation, Durability

• Atomicity ensures all operations in a transaction are completed successfully or none at all

• Consistency ensures database remains in a valid state before and after transaction

• Isolation ensures transactions are independent and do not inter...read more
Answered by AI
Add your answer

Round duration - 45 Minutes
Round difficulty - Easy

The interviewer was friendly. Explained the question well. Allowed to use any preferred platform to write code.

Q1. 
Best Time to Buy and Sell Stock Problem Statement
Given an array prices representing the prices of a stock where each element indicates the price at a given minute, determine the maximum profit you can ac...read more
Ans. 
Find the maximum profit by buying and selling a stock once based on given prices.
• Iterate through the prices array and keep track of the minimum price seen so far and the maximum profit achievable.

• Calculate the profit by subtracting the current price from the minimum price and update the maximum profit if needed.

• Return the maximum profit, ensuring it is not negative.

• Example: prices = [2, 100, 150, 120], Buy at 2, sell a
Answered by AI
View 1 more answer

Round duration - 60 Minutes
Round difficulty - Medium

Q1. 
Dance Team Pairing Challenge
Imagine you are helping Ninja, a dance coach, who needs to form dance pairs from the available boys and girls in a studio. Given the number of boys N, the number of girls M, a...read more
Ans. 
The challenge involves forming dance pairs from available boys and girls based on potential pairings to maximize the number of pairs.
• Parse the input to get the number of test cases, boys, girls, and potential pairings.

• Iterate through the potential pairings and form pairs based on the given indexes.

• Output '1' if a set of maximum possible pairs is returned, else output '0'.

• There can be multiple valid configurations of pa...read more
Answered by AI
Add your answer
Q2. What is the difference between multiprocessing and multithreading?
Ans. 
Multiprocessing involves multiple processes running concurrently, while multithreading involves multiple threads within a single process.
• Multiprocessing utilizes multiple processes to execute tasks simultaneously.

• Multithreading involves multiple threads within a single process sharing the same memory space.

• Multiprocessing is typically used for CPU-bound tasks, while multithreading is more suitable for I/O-bound tasks.

• E...read more
Answered by AI
Add your answer

Round duration - 60 Minutes
Round difficulty - Hard

Q1. Can you describe the classes and functions involved in the design of a system like Splitwise?
Ans. 
Splitwise is a system for managing shared expenses among groups of people.
• Classes: User, Expense, Group

• Functions: addExpense(), settleUp(), calculateBalance()
Answered by AI
Add your answer

Round duration - 60 Minutes
Round difficulty - Hard

Leetcode simple problems. But with coding

C++ basics and operating systems basic

Q1. C++ templates and advanced c++
Add your answer
Q2. Low level design
Add your answer

DSA medium level questions were asked.

Q1. Design MakeMyTrip kind of application.
Ans. 
MakeMyTrip is a travel booking application that allows users to book flights, hotels, and holiday packages.
• Include features like flight/hotel search, booking, payment gateway integration, and user profiles.

• Implement filters for search results, reviews/ratings for hotels, and notifications for booking updates.

• Integrate maps for location tracking, weather forecasts, and customer support chatbot.

• Offer discounts, loyalty p...read more
Answered by AI
Add your answer
Q2. Oops based questions.
Add your answer

Tell me about a time where you resolve a conflict

Reverse a string, identify anagram