OLX SDE-2 Interview Questions and Answers

2 Coding questions both easy-medium level.
1. Based on the Sliding Window
2. Graph Question

A simple feed platform, which supports some basic functionality. The main point of this round was to see how well a candidate architect the application.

Q1. Basic discussion around the project & past experience.
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Q2. Design a 2/3rd Winning game.
Ans. 
Design a 2/3rd Winning game.
• The game should have a clear win condition

• The win condition should be achievable in 2/3rd of the total game time

• The game should have a balanced difficulty level

• The game should have a clear feedback system for the player

• Examples: Chess, Tic Tac Toe, Connect Four
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Q3. Discussion around how would you scale a application.
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Q4. NoSQL vs SQL difference
Ans. 
NoSQL is non-relational and schema-less while SQL is relational and has a fixed schema.
• NoSQL databases are horizontally scalable and can handle large amounts of unstructured data.

• SQL databases are vertically scalable and are better suited for structured data with complex relationships.

• NoSQL databases are often used in web applications, while SQL databases are commonly used in enterprise applications.

• Examples of NoSQL d...read more
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Q1. Hiring Manager round
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Q2. Basic discussions around past work, past processes, what are your future aspirations etc.
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Q3. This round was mainly for seeing the culture fitment of a candidate.
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Round duration - 120 minutes
Round difficulty - Medium

it was morning 10AM-12
friendly environment given by the interviewer
It was machine coding round, the problem statement was to create a online food ordering system with various features.
interviewer was good and supportive

Round duration - 60 Minutes
Round difficulty - Medium

timing noon 3-4Pm
good interviewer 
interviwer was giving hints if required and all

Q1. 
Problem: Sort an Array of 0s, 1s, and 2s
Given an array/list ARR consisting of integers where each element is either 0, 1, or 2, your task is to sort this array in increasing order.
Input:
The input sta...read more
Ans. 
Sort an array of 0s, 1s, and 2s in increasing order.
• Use a three-pointer approach to sort the array in a single pass.

• Initialize three pointers for 0, 1, and 2 values and iterate through the array.

• Swap elements based on the values encountered to achieve the sorted array.

• Example: Input array [0, 2, 1, 1] should be sorted as [0, 1, 1, 2].
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Q2. 
Maximum Path Sum Problem Statement
You are given an n-ary tree consisting of 'N' nodes. Your task is to determine the maximum sum of the path from the root to any leaf node.
Example:
Input:
For the giv...read more
Ans. 
Find the maximum sum of the path from the root to any leaf node in an n-ary tree.
• Traverse the tree from root to leaf nodes, keeping track of the sum along the path.

• At each node, calculate the sum of the path from the root to that node and update the maximum sum found so far.

• Consider using depth-first search (DFS) to traverse the tree efficiently.

• Handle cases where nodes are absent (-1) by appropriately updating the pat...read more
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Round duration - 90 Minutes
Round difficulty - Easy

evening 4-5:30PM
interviewer was good
very supportive and giving hints

Q1. Design a system for BookMyShow to allow users to book movie tickets.
Ans. 
Design a system for BookMyShow to allow users to book movie tickets.
• Create a user-friendly website and mobile app for users to browse movies and showtimes.

• Implement a secure payment gateway for users to purchase tickets online.

• Develop a database to store movie information, showtimes, and user bookings.

• Include features like seat selection, ticket confirmation, and booking history for users.

• Integrate with cinema partners
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Round duration - 40 Minutes
Round difficulty - Easy

4-5 PM
Interviewer was good

Q1. Why should we hire you?
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Q2. Why are you considering a switch in your job?
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This was a machine coding round in which we have to implement an online cab booking service.

Standard problem solving round consists of two questions on tree and dp.

Hacker Rank test - 4 rounds (Coding, System Design and Work style assessment, Leadership Principles)

Q1. LC - 2 medium difficulty questions
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Q2. System design discussion
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Round duration - 45 Minutes
Round difficulty - Easy

Q1. 
Balanced Parentheses Combinations
Given an integer N representing the number of pairs of parentheses, find all the possible combinations of balanced parentheses using the given number of pairs.
Explanati...read more
Ans. 
Generate all possible combinations of balanced parentheses for a given number of pairs.
• Use recursion to generate all possible combinations of balanced parentheses.

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

• Terminate recursion when the number of open and close parentheses used equals the given number of pairs.
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Round duration - 45 Minutes
Round difficulty - Easy

Q1. 
Boundary Traversal of Binary Tree Problem Statement
You are given a binary tree of integers. Your task is to print the boundary nodes of this binary tree in an anti-clockwise direction starting from the ro...read more
Ans. 
Print the boundary nodes of a binary tree in an anti-clockwise direction starting from the root node.
• Traverse the left boundary nodes from top to bottom

• Traverse the leaf nodes from left to right

• Traverse the right boundary nodes from bottom to top

• Handle cases where nodes are null (-1)

• Print the boundary nodes in the specified order
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Round duration - 45 Minutes
Round difficulty - Easy

Q1. 
Bottom View of Binary Tree Problem Statement
Given a binary tree, the task is to print its bottom view from left to right. Assume the left and the right child nodes make a 45-degree angle with their paren...read more
Ans. 
The task is to print the bottom view of a binary tree from left to right.
• Traverse the binary tree in level order and keep track of the horizontal distance of each node from the root.

• For each horizontal distance, update the node value in a map to get the bottom view nodes.

• Print the values of the map in sorted order of horizontal distance to get the bottom view sequence.
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Round duration - 45 minutes
Round difficulty - Easy

Bar raiser

Q1. Can you design a parking lot using low-level design (LLD) and a tiny URL service using high-level design (HLD)?
Ans. 
Yes, I can design a parking lot using LLD and a tiny URL service using HLD.
• For designing a parking lot using LLD, we can create classes like ParkingLot, ParkingSpot, Vehicle, etc. with their respective attributes and methods.

• For designing a tiny URL service using HLD, we can focus on scalability, fault tolerance, and performance. We can use techniques like sharding, caching, load balancing, etc.

• In the parking lot LLD, ...read more
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Round duration - 60 mins
Round difficulty - Medium

3 Questions were asked : 2- Trees and 1- Stack.
Interviewer was not receptive at all, I think this was a stress interview.

Q1. 
Validate Binary Search Tree Problem Statement
Given a binary tree with 'N' nodes, determine if it is a Binary Search Tree (BST). Return true if the tree is a BST, otherwise return false.
Example:
Input:
...read more
Ans. 
Validate if a given binary tree is a Binary Search Tree (BST) or not.
• Check if the left subtree of a node contains only nodes with values less than the node's value.

• Check if the right subtree of a node contains only nodes with values greater than the node's value.

• Ensure that both the left and right subtrees are also binary search trees.

• Traverse the tree in-order and check if the elements are in sorted order.

• Use a recurs...read more
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Q2. 
Binary Tree Conversion to Greater Tree
Given a binary tree with 'N' nodes, transform it into a Greater Tree. In this transformation, each node's value should be updated to the sum of its original value and...read more
Ans. 
Convert a binary tree into a Greater Tree by updating each node's value to the sum of its original value and the values of all nodes greater than or equal to it.
• Traverse the tree in reverse inorder (right, root, left) to update the nodes' values.

• Keep track of the sum of nodes visited so far to update each node's value.

• Recursively update the node's value by adding the sum of greater nodes to it.
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Q3. 
Check for Valid Parentheses
You are given a string STR containing only the characters "{", "}", "(", ")", "[", and "]". Your task is to determine if the parentheses in the string are balanced.
Input:
Th...read more
Ans. 
Check if the given string containing only parentheses is balanced or not.
• Use a stack to keep track of opening parentheses.

• Iterate through the string and push opening parentheses onto the stack.

• When a closing parenthesis is encountered, pop from the stack and check if it matches the corresponding opening parenthesis.

• If at the end the stack is empty, return 'YES' else return 'NO'.
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Round duration - 60 mins
Round difficulty - Hard

2 Coding Questions were asked, both were of Medium to Hard Difficulty.

Q1. 
Flatten Multi-Level Linked List
You are given a multi-level linked list containing 'N' nodes. Each node has 'next' and 'child' pointers that may or may not point to other nodes. Your task is to flatten th...read more
Ans. 
Flatten a multi-level linked list into a single linked list by rearranging the pointers.
• Traverse the linked list and whenever a node has a child, flatten the child list and append it after the current node.

• Update the 'next' and 'child' pointers accordingly while flattening the list.

• Continue this process until all nodes are rearranged into a single linked list.
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Q2. 
Rain Water Trapping Problem Statement
Given an array/list ARR of size N, representing an elevation map where each element ARR[i] denotes the elevation of the i-th bar. Your task is to calculate and print ...read more
Ans. 
Calculate the total amount of rainwater that can be trapped between given elevations in an elevation map.
• Use two-pointer approach to keep track of left and right boundaries.

• Calculate the trapped water by finding the minimum of left_max and right_max for each bar.

• Subtract the current bar's height from the minimum of left_max and right_max to get the trapped water.

• Keep updating the left_max and right_max as you iterate t
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Round duration - 90 minutes
Round difficulty - Medium

Timing : It was late night.
How the interviewer was? He was very tired and I had to lead the discussion.
1 System Design Question was asked, had to provide - HLD, LLD, APIs.

Round duration - 60 minutes
Round difficulty - Medium

Timing : Afternoon
How was the environment? Home
Interviewer : Hiring Manager

Q1. Hiring Manager Discussion
Discussed about past Projects, challenges faced, disagreements with manager, why looking for job switch,  leadership principle questions were asked.
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Round duration - 60 minutes
Round difficulty - Medium

Timing - Afternoon
How was the environment? Home
How the interviewer was? Very Helpful Interviewer
Bar raiser Round - Coding + System Design + Leadership principle

Round duration - 60 minutes
Round difficulty - Easy

Started with brief intro(5 mins) about interviewer.
Problem solving question, question was already there in the codelink shared by the interviewer. He explained the problem again with sample testcase. 
The Interviewer was friendly.

Q1. 
Course Schedule II Problem Statement
You are provided with a number of courses 'N', some of which have prerequisites. There is a matrix named 'PREREQUISITES' of size 'M' x 2. This matrix indicates that fo...read more
Ans. 
Given courses with prerequisites, determine a valid order to complete all courses.
• Create a graph with courses as nodes and prerequisites as edges.

• Use topological sorting to find a valid order to complete all courses.

• Return an empty list if it's impossible to complete all courses.
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Q2. Design a live video broadcast platform.
Ans. 
Design a live video broadcast platform.
• Implement video streaming functionality using protocols like RTMP or WebRTC

• Include features for live chat, reactions, and audience engagement

• Ensure scalability and reliability by using cloud services like AWS or Azure

• Provide analytics for viewership data and user engagement

• Integrate monetization options such as ads or subscriptions
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Round duration - 90 minutes
Round difficulty - Medium

There were 2 interviewers(India HM and Seattle HM), India HM was just shadowing and didn't ask any questions. Started with intro, he asked in detail about what I do at current company(10 mins). Again, jumped into LP's(25 mins).

Q1. 
Distance Between Two Nodes in a Binary Tree
Given a binary tree and the values of two distinct nodes, determine the distance between these two nodes in the tree. The distance is defined as the minimum num...read more
Ans. 
Calculate the distance between two nodes in a binary tree.
• Traverse the tree to find the paths from the root to each node

• Find the lowest common ancestor of the two nodes

• Calculate the distance by summing the distances from each node to the common ancestor
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Q2. 
Median in a Stream Problem Statement
Your task is to determine the median of integers as they are read from a data stream. The median is the middle value in the ordered list of numbers. If the list length...read more
Ans. 
Find median of integers in a data stream as they are read.
• Use two heaps - max heap for lower half of numbers and min heap for upper half.

• Keep the size of two heaps balanced to find the median efficiently.

• Handle even and odd number of elements separately to calculate median.

• Return vector of medians after each element is read from the stream.
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Round duration - 60 minutes
Round difficulty - Medium

There was only 1 interviewer in this round and We had no video sharing in this round so it was becoming difficult to understand each other.

Round duration - 90 minutes
Round difficulty - Easy

I got a call from an Amazon recruiter regarding the schedule of the test . She sent me the coding test link which I had to complete within a week. we can attempt the online test anytime, when we have time for a period of 90 minutes at a stretch.Once I completed the test, I got a call to schedule the interviews in two weeks.

Q1. 
Shopping Options Problem Statement
Given arrays representing the costs of pants, shirts, shoes, and skirts, and a budget amount 'X', determine the total number of valid combinations that can be purchased ...read more
Ans. 
Determine total number of valid shopping combinations within budget
• Iterate through all possible combinations of items from each array

• Check if the total cost of the combination is within the budget

• Return the count of valid combinations
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Q2. 
Max Submatrix Problem Statement
You are provided with a matrix MAT consisting of integers, with dimensions N x M (i.e., N rows and M columns). Your objective is to determine the maximum sum submatrix with...read more
Ans. 
Find the maximum sum submatrix in a given matrix.
• Iterate over all possible submatrices and calculate their sums

• Use Kadane's algorithm to find the maximum sum subarray in each row

• Combine the sums of rows to find the maximum sum submatrix
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Round duration - 60 minutes
Round difficulty - Medium

Virtual interview occured in amazon chime.The interviewer gave his introduction, asked me a bit on the kind of projects I’ve worked on. Then he started with a data structure problem.

Q1. 
Clone a Binary Tree with Random Pointers
Given a binary tree where each node has pointers to its left, right, and a random node, create a deep copy of the binary tree.
Input:
The first line contains an ...read more
Ans. 
Clone a binary tree with random pointers and verify if cloning was successful by printing inorder traversal.
• Create a deep copy of the binary tree with random pointers.

• Print the inorder traversal of the cloned binary tree.

• Verify cloning success by printing '1' if successful, '0' otherwise.
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Q2. 
The Ninja Port Problem
Ninja is in a city with 'N' colonies, where each colony contains 'K' houses. He starts at house number "sourceHouse" in colony number "sourceColony" and wants to reach house number ...read more
Ans. 
The Ninja Port Problem involves finding the minimum time for a ninja to travel between colonies and houses using secret paths and teleportation.
• Calculate the minimum time considering teleportation within colonies, traveling between colonies, and secret paths.

• Keep track of the number of secret paths used and ensure they are one-way.

• Consider the constraints provided to optimize the solution.

• Example: N=2, K=3, S=1, P=1, s...read more
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Round duration - 75 Minutes
Round difficulty - Easy

This was taken by an engineering manager who asked me regarding my projects for around 10-15 mins and then we moved to a system design problem. He asked me to design Slack messenger.
I started by listing the functional and non-functional requirements(on which he questioned me a bit), then I moved to draw the high-level architecture. The components which I drew were the clients, gateway service(LB + authentication, etc), Messaging Service, User Service, Web Socket Manager service, Fan Out service(I added this for the group messages thing, but he didn’t interrogate much on that).

He asked me what would be the schema of my messages table and the scenarios in which the recipient user is online/offline.Also asked about the partitioning key and primary key of the 2-3 tables which I had made

Round duration - 60 minutes
Round difficulty - Hard

BarRaiser Round

This was again taken by an engineering manager who discussed my projects in depth for around 20-25 mins. In the remaining time he asked me 2 dsa questions. (Yes I too was surprised that he didn’t ask anything regarding design).

In almost all the rounds, I was asked questions related to Amazon Leadership principles, so do make sure you go through those before sitting for the interview process. You can refer to this link(https://www.codingninjas.com/codestudio/problem-lists/top-amazon-coding-interview-questions) for practicing the same, I found it useful. 
In the design rounds, interviewer doesn’t expect the most ideal answer from you and unless your choice of technology is outrageously wrong, he won’t pinpoint that.

Q1. 
Group Anagrams Together
Given an array/list of strings STR_LIST, group the anagrams together and return each group as a list of strings. Each group must contain strings that are anagrams of each other.
E...read more
Ans. 
Group anagrams together in a list of strings.
• Iterate through the list of strings and sort each string to group anagrams together.

• Use a hashmap to store the sorted string as key and the original string as value.

• Return the values of the hashmap as the grouped anagrams.
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Q2. 
Add Two Numbers Represented by Linked Lists
Your task is to find the sum list of two numbers represented by linked lists and return the head of the sum list.
Explanation:
The sum list should be a linked...read more
Ans. 
Add two numbers represented by linked lists and return the head of the sum list.
• Traverse both linked lists simultaneously while keeping track of carry from previous sum

• Create a new linked list to store the sum of the two numbers

• Handle cases where one linked list is longer than the other by padding with zeros

• Update the sum and carry values accordingly while iterating through the linked lists
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