John Snow, Inc Interview Questions and Answers

Round duration - 60 minutes
Round difficulty - Medium

Started with the discussion why do you want to join DE shaw. Since prior to my current company, I was working with a finance based company so they asked why do you want to come back to finance base company. So they asked my about debentures, bonds, mortgages and other financial jargons.
Then they asked questions related to DSA, OS etc.

Q1. 
Find Triplets with Given Sum Problem Statement
Given an array ARR consisting of N integers, your task is to identify all distinct triplets within the array that sum up to a given integer K.
Explanation:
...read more
Ans. 
The task is to find all distinct triplets in an array that sum up to a given integer.
• Iterate through the array and use nested loops to find all possible triplets.

• Use a set to store unique triplets and avoid duplicates.

• Check if the sum of the triplet equals the target sum.

• Return the list of valid triplets or -1 if no such triplet exists.
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Q2. 
Minimum Number of Platforms Problem
Your task is to determine the minimum number of platforms required at a railway station so that no train has to wait.
Explanation:
Given two arrays:
• AT - represent...read more
Ans. 
Determine the minimum number of platforms needed at a railway station so that no train has to wait.
• Sort the arrival and departure times arrays in ascending order.

• Initialize two pointers for arrival and departure times, and a variable to keep track of the maximum number of platforms needed.

• Increment the platform count when a train arrives and decrement when a train departs.

• Update the maximum platform count as needed.

• Ret...read more
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Q3. What is the difference between internal fragmentation and external fragmentation in operating systems?
Ans. 
Internal fragmentation occurs when memory is allocated in fixed-size blocks, leading to wasted space within a block. External fragmentation occurs when free memory is fragmented into small chunks, making it difficult to allocate contiguous blocks.
• Internal fragmentation is caused by allocating fixed-size blocks of memory, leading to wasted space within a block.

• External fragmentation occurs when free memory is fragmented...read more
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Q4. Write an SQL query to find the employee with the nth highest salary.
Ans. 
SQL query to find the employee with the nth highest salary
• Use the ORDER BY clause to sort salaries in descending order

• Use the LIMIT clause to select the nth highest salary

• Join the employee table with the salary table to get the employee details
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Q5. How is Java platform independent?
Ans. 
Java is platform independent due to its bytecode and JVM.
• Java code is compiled into bytecode, which can run on any platform with a Java Virtual Machine (JVM)

• JVM acts as an intermediary between the Java code and the underlying platform, ensuring portability

• Developers write code once and it can be executed on any platform that has a compatible JVM

• Examples: Windows, macOS, Linux all can run Java programs without any chang
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Q6. What is the difference between an abstract class and an interface in Java?
Ans. 
Abstract class can have both abstract and non-abstract methods, while interface can only have abstract methods.
• Abstract class can have constructor, member variables, and methods with implementation.

• Interface can only have abstract methods and constants.

• A class can implement multiple interfaces but can only extend one abstract class.

• Example: Abstract class - Animal with abstract method 'eat' and non-abstract method 'sle
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Round duration - 45 minutes
Round difficulty - Medium

Technical Interview round with questions on Core Subjects mainly.

Q1. What is the difference between C and C++?
Ans. 
C is a procedural programming language while C++ is an object-oriented programming language with features like classes and inheritance.
• C is a procedural programming language, while C++ is a multi-paradigm language with support for object-oriented programming.

• C does not support classes and objects, while C++ does.

• C uses structures for data organization, while C++ uses classes for data organization.

• C does not have featur...read more
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Q2. What is a virtual function?
Ans. 
A virtual function is a function in a base class that is declared using the keyword 'virtual' and can be overridden by a function with the same signature in a derived class.
• Virtual functions allow for dynamic polymorphism in object-oriented programming.

• They are used to achieve runtime binding and enable the implementation of the function to be determined during runtime.

• Example: virtual void display() = 0; in a base cla...read more
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Q3. What is inheritance in object-oriented programming?
Ans. 
Inheritance is a concept in object-oriented programming where a class inherits properties and behaviors from another class.
• Allows for code reusability by creating a new class based on an existing class

• Child class inherits attributes and methods of the parent class

• Can have multiple levels of inheritance (e.g. grandparent, parent, child classes)

• Example: class Dog extends Animal, where Dog inherits properties and methods
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Q4. What is the difference between a Default Constructor and a Copy Constructor in Object-Oriented Programming?
Ans. 
Default Constructor is a constructor that is automatically called when an object is created without any arguments. Copy Constructor is used to create a new object as a copy of an existing object.
• Default Constructor has no parameters, while Copy Constructor takes an object of the same class as a parameter.

• Default Constructor initializes the object with default values, while Copy Constructor creates a new object with the...read more
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Round duration - 40 minutes
Round difficulty - Easy

Technical Interview round with questions on Core Subjects mainly.

Q1. What are the types of access modifiers in C++?
Ans. 
The types of access modifiers in C++ are public, private, and protected.
• Public: accessible from outside the class

• Private: only accessible within the class

• Protected: accessible within the class and its subclasses
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Q2. What is the difference between DDL (Data Definition Language) and DML (Data Manipulation Language)?
Ans. 
DDL is used to define the structure of database objects, while DML is used to manipulate data within those objects.
• DDL is used to create, modify, and delete database objects like tables, indexes, etc.

• DML is used to insert, update, delete, and retrieve data from database tables.

• DDL statements include CREATE, ALTER, DROP, TRUNCATE, etc.

• DML statements include INSERT, UPDATE, DELETE, SELECT, etc.
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Q3. What are the key differences between a Primary Key and a Unique Key in a database management system?
Ans. 
Primary Key uniquely identifies each record in a table, while Unique Key ensures that all values in a column are distinct.
• Primary Key does not allow NULL values, while Unique Key allows one NULL value.

• A table can have only one Primary Key, but multiple Unique Keys.

• Primary Key is automatically indexed, while Unique Key may or may not be indexed.

• Primary Key is used to establish relationships between tables, while Unique ...read more
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Q4. What is the difference between DBMS and RDBMS?
Ans. 
DBMS is a software system that manages databases, while RDBMS is a type of DBMS that stores data in a structured format using tables.
• DBMS stands for Database Management System, which is a software system that allows users to interact with a database.

• RDBMS stands for Relational Database Management System, which is a type of DBMS that stores data in a structured format using tables with rows and columns.

• RDBMS uses Struct...read more
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Q5. What are the objectives of normalization in database management systems?
Ans. 
Normalization in database management systems aims to reduce data redundancy, improve data integrity, and optimize database performance.
• Eliminate data redundancy by breaking down data into smaller tables

• Reduce update anomalies by ensuring data is stored in a logical and consistent manner

• Improve data integrity by enforcing referential integrity constraints

• Optimize database performance by reducing the amount of redundant
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Round duration - 30 minutes
Round difficulty - Easy

HR round with typical behavioral problems.

Round duration - 40 minutes
Round difficulty - Medium

The test has 3 sections :
1. Quantitative contains 20 questions. These questions are easy to me. If u practice quant, you can easily solve it.

2. Technical contains 20 questions. Some questions are on C , C++ , java outputs. Remaining are based on DSA. If u Know all the time complexities of algorithms then it is easy . In technical they gave one DSA problem.
 

3. Essay: They have given one picture and then asked to write an essay by seeing the picture.

Q1. 
Dijkstra's Algorithm for Shortest Path
Given an undirected graph with 'V' vertices labeled from 0 to V-1 and 'E' edges. Each edge has a weight that represents the distance between two nodes 'X' and 'Y'.
...read more
Ans. 
Dijkstra's algorithm is used to find the shortest path distances from a source node to all other vertices in a graph.
• Dijkstra's algorithm is a greedy algorithm that finds the shortest path from a source node to all other nodes in a graph.

• It uses a priority queue to select the node with the smallest distance and relaxes its neighbors.

• The algorithm maintains a distance array to keep track of the shortest distances from t...read more
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Round duration - 60 minutes
Round difficulty - Easy

3 persons are there in panel. They are asking questions on basic concepts in subjects and some advanced concepts.

Q1. What is a Daemon Process?
Ans. 
A daemon process is a background process that runs continuously to perform system tasks.
• Daemon processes do not have a controlling terminal.

• They typically run in the background and perform tasks such as managing hardware devices, network services, or system maintenance.

• Examples of daemon processes include cron, sshd, and apache.

• Daemon processes are often started during system boot and run until the system is shut down.
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Q2. What are the advantages of system calls in operating systems?
Ans. 
System calls provide a way for user-level processes to interact with the operating system kernel.
• System calls allow user programs to request services from the operating system.

• They provide a secure and controlled way for applications to access system resources.

• System calls enable processes to perform tasks such as file operations, network communication, and process management.

• Examples of system calls include open(), re
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Q3. Can you explain the select() system call?
Ans. 
The select() system call is used in Unix-like operating systems to monitor multiple file descriptors for activity.
• select() allows a program to wait for multiple I/O operations to complete on multiple file descriptors.

• It takes three sets of file descriptors as arguments - readfds, writefds, and errorfds.

• The function will block until an activity is detected on one of the file descriptors or until a timeout occurs.

• select(...read more
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Round duration - 45 minutes
Round difficulty - Medium

Technical Interview round with questions on DSA.

Q1. 
Binary Tree Mirror Conversion Problem
Given a binary tree, your task is to convert it into its mirror tree.
A binary tree is a data structure where each parent node has at most two children.
The mirror ...read more
Ans. 
Convert a binary tree into its mirror tree by interchanging left and right children of non-leaf nodes.
• Traverse the binary tree in postorder fashion and swap the left and right children of each non-leaf node.

• Use recursion to solve the problem efficiently.

• Remember to handle null child nodes represented by -1 in the input.
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Q2. 
Maximum Sum Subarray Problem Statement
Given an array of integers, find the maximum sum of any contiguous subarray within the array.
Example:
Input:
array = [34, -50, 42, 14, -5, 86]
Output:
137
Exp...read more
Ans. 
Find the maximum sum of any contiguous subarray within an array in O(N) time complexity.
• Iterate through the array and keep track of the maximum sum of subarrays encountered so far

• At each index, decide whether to include the current element in the subarray or start a new subarray

• Update the maximum sum if a new maximum is found

• Example: For array [34, -50, 42, 14, -5, 86], the maximum sum subarray is [42, 14, -5, 86] with
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Round duration - 45 minutes
Round difficulty - Hard

Technical Interview round with questions on DSA.

Q1. 
Find the Largest BST Subtree in a Given Binary Tree
Your task is to determine the size of the largest subtree of a given binary tree which is also a Binary Search Tree (BST).
BST Definition:
• The left...read more
Ans. 
Find the size of the largest subtree in a binary tree that is also a Binary Search Tree (BST).
• Traverse the binary tree in a bottom-up manner to check if each subtree is a BST.

• Keep track of the size of the largest BST subtree encountered so far.

• Use the properties of a BST to determine if a subtree is a valid BST.

• Consider edge cases such as empty tree or single node tree.

• Example: For input 1 2 3 4 -1 5 6 -1 7 -1 -1 -1 -1
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Q2. 
Next Higher and Previous Lower Number with Same Number of Set Bits
Given a positive integer n, the task is to find the next smallest integer and the previous largest integer that contain the exact same nu...read more
Ans. 
Find next smallest and previous largest integers with same number of set bits as given integer.
• Count the number of set bits in the given integer using bitwise operations.

• For next smallest integer, find the next number with same number of set bits by iterating through numbers greater than the given integer.

• For previous largest integer, find the previous number with same number of set bits by iterating through numbers sm
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