Pinterest Software Developer Interview Questions and Answers

Round duration - 60 minutes
Round difficulty - Medium

This was a online test round where I was given 3 DSA questions to be solved in 60 minutes.

Q1. 
Count Ways to Reach the N-th Stair Problem Statement
You are provided with a number of stairs, and initially, you are located at the 0th stair. You need to reach the Nth stair, and you can climb one or tw...read more
Ans. 
The problem involves finding the number of distinct ways to climb to the Nth stair by taking one or two steps at a time.
• Use dynamic programming to solve the problem efficiently.

• The number of ways to reach the Nth stair is the sum of the number of ways to reach the (N-1)th stair and the (N-2)th stair.

• Handle base cases for N=0 and N=1 separately.

• Consider using modulo 10^9+7 to avoid overflow in calculations.
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Q2. 
Optimal Strategy for a Coin Game
You are playing a coin game with your friend Ninjax. There are N coins placed in a straight line.
Here are the rules of the game:
1. Each coin has a value associated wit...read more
Ans. 
The problem involves finding the optimal strategy to accumulate the maximum amount in a coin game with specific rules.
• Start by understanding the rules of the game and how players take turns to choose coins.

• Consider the scenario where both players play optimally to maximize winnings.

• Iterate through different strategies to determine the best approach for selecting coins.

• Keep track of the total winnings accumulated by eac...read more
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Q3. 
Generate All Parentheses Combinations
Given an integer N, your task is to create all possible valid parentheses configurations that are well-formed using N pairs. A sequence of parentheses is considered w...read more
Ans. 
Generate all possible valid parentheses configurations using N pairs.
• Use backtracking to generate all possible combinations of parentheses.

• Keep track of the number of open and close parentheses used.

• Add '(' if there are remaining open parentheses, and add ')' if there are remaining close parentheses.

• Base case: when the length of the generated string is 2*N, add it to the result array.
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Round duration - 90 minutes
Round difficulty - Hard

Online coding round , 2-3 questions. Level - med-hard

Q1. 
Fibonacci Number Verification
Identify if the provided integer 'N' is a Fibonacci number.
A number is termed as a Fibonacci number if it appears in the Fibonacci sequence, where each number is the sum of...read more
Ans. 
Check if a given number is a Fibonacci number or not.
• Iterate through the Fibonacci sequence until you find a number greater than or equal to the given number.

• Check if the given number matches the Fibonacci number found in the sequence.

• If the given number matches, output 'YES'; otherwise, output 'NO'.
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Q2. 
Break The Board Problem Statement
Your task is to break a board of given dimensions 'L' by 'W' into 'L' * 'W' smaller squares, ensuring the total cost of breaking is minimized.
Input:
The first line con...read more
Ans. 
Minimize cost of breaking a board into smaller squares by optimizing horizontal and vertical cuts.
• Iterate through all possible horizontal and vertical cuts to find the minimum cost

• Use dynamic programming to store and reuse subproblem solutions

• Calculate the cost of breaking the board into smaller squares based on the given dimensions and cut costs
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Round duration - 60 Minutes
Round difficulty - Medium

2 interviewers , coding problem, no compilation.

Q1. 
Minimum Number Of People To Teach
Ninja has started a social networking site called Ninjas Space. The platform consists of ‘N’ users and supports ‘M’ different languages. Users communicate if they know at...read more
Ans. 
Determine the minimum number of users to teach a common language so all friends can communicate on a social networking site.
• Create a graph where users are nodes and friendships are edges.

• Find connected components in the graph.

• For each connected component, determine the minimum number of users to teach a common language.

• Return the sum of minimum users needed for all connected components.
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Round duration - 40 minutes
Round difficulty - Medium

CS fundamentals. Tough round. 40 mins interview. 1 interviewer from USA

Round duration - 40 minutes
Round difficulty - Hard

System Design Problem

Q1. Design a Facebook-like application.
Ans. 
Design a Facebook-like application for social networking.
• User profiles with personal information and photos

• News feed displaying posts from friends

• Ability to like, comment, and share posts

• Friend requests and messaging functionality

• Groups and events for community engagement
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Leetcode similar coding tests

Q1. How do you test the feasibility of an idea?
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Q2. Explain the process of calling an API
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Q1. Find the possible ways to get the target score
Ans. 
There are multiple ways to achieve the target score in a game or competition.
• Incrementing by 1 point each time until reaching the target score

• Combining different point values to reach the target score

• Using bonus points or multipliers to reach the target score
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Round duration - 60 Minutes
Round difficulty - Medium

This was a Data Structures and Algorithms round with preety good questions . I was expected to come up with an efficient approach and code it as well .

Q1. 
K Closest Points to Origin Problem Statement
Your house is located at the origin (0,0) of a 2-D plane. There are N neighbors living at different points on the plane. Your goal is to visit exactly K neighb...read more
Ans. 
Find the K closest points to the origin in a 2-D plane using Euclidean Distance.
• Calculate the Euclidean Distance of each point from the origin

• Sort the points based on their distances

• Return the first K points as the closest neighbors
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Q2. 
Power Set Generation
Given a sorted array of 'N' integers, your task is to generate the power set for this array. Each subset of this power set should be individually sorted.
A power set of a set 'ARR' i...read more
Ans. 
Generate the power set of a sorted array of integers with individually sorted subsets.
• Use recursion to generate all possible subsets by including or excluding each element in the array.

• Sort each subset before adding it to the power set.

• Handle base cases for empty array and single element array.

• Ensure the subsets are unique by using a set data structure.

• Time complexity can be exponential due to the nature of generating
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Round duration - 50 Minutes
Round difficulty - Hard

This was also a DSA round where I was asked to code only one of the questions but I eventually ended up coding both as I had some spare time and explained my approches very smoothly to the interviewer . This round went preety well .

Q1. 
Roman Numeral to Integer Conversion
Convert a string representing a Roman numeral into its integer equivalent and return the result.
Explanation:
Roman numerals are represented by seven different symbol...read more
Ans. 
Convert a Roman numeral string to its integer equivalent.
• Create a mapping of Roman numeral symbols to their integer values.

• Iterate through the input string and add the corresponding integer values.

• Handle cases where subtraction is needed (e.g., IV = 4, IX = 9).
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Q2. 
Pair Sum Problem Statement
You are given an integer array 'ARR' of size 'N' and an integer 'S'. Your task is to find and return a list of all pairs of elements where each sum of a pair equals 'S'.
Note:
...read more
Ans. 
Find pairs of elements in an array that sum up to a given value, sorted in a specific order.
• Iterate through the array and for each element, check if the complement (S - current element) exists in a hash set.

• If the complement exists, add the pair to the result list.

• Sort the result list based on the criteria mentioned in the question.

• Return the sorted list of pairs.
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Round duration - 50 Minutes
Round difficulty - Medium

This was also a DSA round with 2 questions . One was implementation heavy and the other was related to recursion and so I handled it carefully so that my code does not run into TLE or Segmentation Fault.

Q1. 
Arithmetic Expression Evaluation Problem Statement
You are provided with a string expression consisting of characters '+', '-', '*', '/', '(', ')' and digits '0' to '9', representing an arithmetic express...read more
Ans. 
Evaluate arithmetic expressions in infix notation with given operators and precedence rules.
• Parse the infix expression to postfix using a stack.

• Evaluate the postfix expression using a stack.

• Handle operator precedence and parentheses while evaluating.

• Ensure no division by zero cases and operands fit in 32-bit integer.
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Q2. 
Remove Duplicates from Sorted Array Problem Statement
You are given a sorted integer array ARR of size N. Your task is to remove the duplicates in such a way that each element appears only once. The outpu...read more
Ans. 
Remove duplicates from a sorted array in-place with O(1) extra memory.
• Use two pointers - one for iterating through the array and another for placing unique elements.

• Compare current element with next element to identify duplicates and skip them.

• Update array in-place by moving unique elements to the front.

• Return the length of the array after removal of duplicates.
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Round duration - 50 Minutes
Round difficulty - Medium

This was a typical System Design round where I was asked about the various features of Facebook and what sort of data structures and algorithms are used in implementing them .

Q1. How does Facebook store likes and dislikes?
Ans. 
Facebook stores likes and dislikes using a combination of databases and algorithms.
• Likes and dislikes are stored in databases such as MySQL or Cassandra.

• Algorithms are used to analyze user behavior and recommend content based on likes and dislikes.

• User interactions with posts, pages, and ads are tracked to determine likes and dislikes.

• Likes and dislikes may also be used to personalize the user's feed and target ads mor
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Q2. How does Facebook implement graph search?
Ans. 
Facebook implements graph search using a graph database to efficiently search for connections between users and their interests.
• Facebook uses a graph database to store connections between users, pages, groups, etc.

• The graph search algorithm traverses the graph to find relevant connections based on user queries.

• It takes into account factors like user relationships, interests, and interactions to provide personalized sea
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Round duration - 50 Minutes
Round difficulty - Medium

This was a preety intense round as I was grilled more on my System Design concepts but eventually I was able to asnwers all the questions with some help from the interviewer.

Q1. What is Hadoop and why is it used?
Ans. 
Hadoop is a framework for distributed storage and processing of large data sets.
• Hadoop is used for storing and processing big data across a distributed network of computers.

• It is based on the MapReduce programming model, which allows for parallel processing of data.

• Hadoop consists of HDFS for storage and YARN for resource management.

• It is used for tasks like data warehousing, log processing, recommendation systems, and...read more
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Q2. How does Facebook Chat work?
Ans. 
Facebook Chat works by using a combination of websockets, long polling, and push technology to deliver real-time messaging.
• Facebook Chat uses websockets for real-time communication between the client and server.

• Long polling is used to check for new messages when websockets are not supported.

• Push technology is used to notify users of new messages even when the chat window is not open.

• Messages are stored in a database an...read more
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Round duration - 60 minutes
Round difficulty - Medium

This was a 1 hour online coding round on hackerrank platform. 3 DSA questions were given.

Q1. 
Stock Trading Maximum Profit Problem
Given the stock prices for 'N' days, your goal is to determine the maximum profit that can be achieved. You can buy and sell the stocks any number of times but can onl...read more
Ans. 
The goal is to determine the maximum profit that can be achieved by buying and selling stocks on different days.
• Iterate through the stock prices and buy on days when the price is lower than the next day, and sell on days when the price is higher than the next day.

• Calculate the profit by summing up the differences between buying and selling prices.

• Repeat the process for each test case and output the maximum profit possi
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Q2. 
Ninja's Complement Problem Statement
Every day, Ninja dreams of a number but does not understand its significance. Upon investigation, Ninja discovers that converting this number to binary, taking its com...read more
Ans. 
Convert a number to binary, take its complement, and convert back to decimal.
• Convert the given number to binary representation

• Take the complement of the binary number by swapping '1's and '0's

• Convert the complement back to decimal representation

• Return the complement number for each test case
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Q3. 
Bit Majority Number Generation
You are given an array arr consisting of N integers. Construct a number from these integers such that, for each bit position, if the quantity of set bits (1s) is greater tha...read more
Ans. 
Generate a number based on majority set bits in given integers array.
• Iterate through each bit position and count set and unset bits

• Construct the resulting number based on majority set bits at each position

• Return the final constructed number
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Round duration - 60 minutes
Round difficulty - Easy

In this round, a DSA question was first asked. Then a lot of discussion regarding the various test cases around this question took place. (e.g, handing of -ve numbers , decimals , overflow etc).
Main concentration was just to get as many as test cases possible and an efficient solution as well.
And Some common question like why you coding in C++?

Q1. 
Sum of Digits Problem Statement
Given an integer 'N', continue summing its digits until the result is a single-digit number. Your task is to determine the final value of 'N' after applying this operation ...read more
Ans. 
Given an integer, sum its digits until a single-digit number is obtained.
• Iteratively sum the digits of the integer until a single-digit number is reached

• Output the final single-digit integer for each test case

• Handle multiple test cases efficiently
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Q2. What are the advantages of Object-Oriented Programming (OOP)?
Ans. 
OOP allows for code reusability, modularity, and easier maintenance.
• Encapsulation: Bundling data and methods together to protect data from outside interference.

• Inheritance: Allows for the creation of new classes based on existing classes, promoting code reuse.

• Polymorphism: Objects can be treated as instances of their parent class, allowing for flexibility and extensibility.

• Modularity: Breaking down a program into small...read more
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Round duration - 60 minutes
Round difficulty - Easy

Again one coding question to be done on shared screen

Q1. 
Counting Triangles in Graphs
Given two graphs – a directed graph DIR_GRAPH and an undirected graph UNDIR_GRAPH – you are tasked with counting the number of triangles in each of the graphs.
Example:
In t...read more
Ans. 
Count the number of triangles in a directed and undirected graph.
• Parse the input to extract vertices, edges, and edges between vertices.

• Implement a function to count triangles in both directed and undirected graphs.

• Return the counts of triangles for each test case.
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Round duration - 60 minutes
Round difficulty - Easy

Technical Interview round with questions based on DSA, UNIX commands , Web Security etc.

Q1. 
Find The Repeating And Missing Number Problem Statement
You are provided with an array nums which contains the first N positive integers. In this array, one integer appears twice, and one integer is missi...read more
Ans. 
Identify the repeating and missing number in an array of first N positive integers.
• Iterate through the array and keep track of the sum of elements and sum of squares to find the missing and repeating numbers.

• Use a set to identify the repeating number and calculate the missing number using the sum formula.

• Handle multiple test cases by looping through each case and applying the same logic.

• Example: For nums = [1, 2, 3, 4,...read more
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Q2. What is the difference between the 'ps' and 'top' commands?
Ans. 
ps displays a snapshot of current processes, while top provides real-time information and allows for interactive management.
• ps displays a static list of processes, while top continuously updates the list

• top allows for interactive management of processes (e.g. killing, renicing), while ps does not

• ps is useful for a quick snapshot of processes, while top is useful for monitoring real-time system activity
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Q3. What is cross-site scripting (XSS)?
Ans. 
Cross-site scripting (XSS) is a type of security vulnerability that allows attackers to inject malicious scripts into web pages viewed by other users.
• XSS occurs when a web application allows users to input unvalidated data that is then displayed on the page without proper encoding.

• There are three main types of XSS: stored XSS, reflected XSS, and DOM-based XSS.

• An example of XSS is when a user inputs a script into a sear...read more
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Q4. How can you prevent SQL injection?
Ans. 
Prevent SQL injection by using parameterized queries, input validation, and escaping user input.
• Use parameterized queries to separate SQL code from user input.

• Validate and sanitize user input to prevent malicious code injection.

• Escape special characters in user input before executing SQL queries.
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Q5. What is the command to kill all running Java processes?
Ans. 
The command to kill all running Java processes is 'pkill -f java'
• Use the 'pkill' command followed by the '-f' flag and 'java' keyword to kill all Java processes

• Be cautious when using this command as it will terminate all running Java processes

• Make sure to have the necessary permissions to kill processes on the system
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Round duration - 60 minutes
Round difficulty - Medium

Questions based on designing. My first project as it was related to Image processing, so he also asked questions related to it.

Q1. Design a complete system for a traffic enforcement camera.
Ans. 
Design a system for a traffic enforcement camera.
• 1. Install cameras at strategic locations to monitor traffic violations.

• 2. Use image recognition software to detect violations such as speeding or running red lights.

• 3. Store captured images and violation data in a secure database.

• 4. Generate automated tickets and send them to violators.

• 5. Implement a user interface for law enforcement to review violations and approve ti
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Q2. How does JSON (JavaScript Object Notation) work?
Ans. 
JSON is a lightweight data interchange format that is easy for humans to read and write and easy for machines to parse and generate.
• JSON is based on key-value pairs, where keys are always strings enclosed in double quotes.

• Data is organized in a hierarchical structure using objects and arrays.

• Example: {"name": "John", "age": 30}

• Example: [{"name": "Alice", "age": 25}, {"name": "Bob", "age": 35}]
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Q3. What software testing frameworks are you familiar with?
Ans. 
I am familiar with testing frameworks such as JUnit, Selenium, and TestNG.
• JUnit

• Selenium

• TestNG
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Q4. What is the best protocol for storing images?
Ans. 
The best protocol for storing images is JPEG (Joint Photographic Experts Group).
• JPEG is a widely used format for storing images due to its high compression ratio and good image quality.

• It supports millions of colors and is compatible with most devices and software.

• Other options include PNG (Portable Network Graphics) for lossless compression and GIF (Graphics Interchange Format) for animations.
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Round duration - 30 minutes
Round difficulty - Easy

This was a typical HR round with questions related to my final year project and some behavioral problems.