Freecharge Backend Developer Interview Questions and Answers

Easy Medium questions

Q1. Asked to design Basic Facebook.
Ans. 
Design a basic social networking platform similar to Facebook, focusing on user profiles, posts, and interactions.
• User Registration: Users can create accounts with email and password.

• User Profiles: Each user has a profile with a photo, bio, and friend list.

• Posts: Users can create text, image, or video posts visible to friends.

• News Feed: A central feed displays posts from friends and followed users.

• Friend Requests: User...read more
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It was about DS Algo and Java garbage collection,multithreading

Q1. Low level design of rate limiter
Ans. 
Rate limiter is a system that controls the rate of traffic sent or received by a network interface.
• Implement a token bucket algorithm to track and limit the rate of requests

• Use a sliding window algorithm to track the number of requests within a specific time frame

• Consider using a distributed rate limiter for scalability and fault tolerance
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Q2. Springboot basics, java basics, future interfaces, basic sql queries
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Data Structures Algorithms

Data Structures and Algorithm

Round duration - 90 minutes
Round difficulty - Medium

Only Java allowed

Q1. 
Sort 0 and 1 Problem Statement
Given an integer array ARR of size N containing only integers 0 and 1, implement a function to sort this array. The solution should scan the array only once without using an...read more
Ans. 
Sort an array of 0s and 1s in linear time without using additional arrays.
• Iterate through the array and maintain two pointers, one for 0s and one for 1s.

• Swap elements at the two pointers based on the current element being 0 or 1.

• Continue this process until the entire array is sorted in place.
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Round duration - 60 minutes
Round difficulty - Easy

Basic intro and coding questions

Q1. 
Valid Perfect Square Check
An integer N is provided, and the task is to determine if N is a perfect square. A perfect square refers to an integer which can be expressed as the square of another integer.
...read more
Ans. 
Implement a function to check if a given integer is a perfect square.
• Create a function that takes an integer as input and returns true if it is a perfect square, false otherwise.

• Iterate through possible square roots of the input integer and check if the square of the root equals the input.

• Return true if a perfect square is found, false otherwise.
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Q2. 
Merge Sort Problem Statement
Given a sequence of numbers 'ARR', your task is to return a sorted sequence of 'ARR' in non-descending order using the Merge Sort algorithm.
Example:
Input:
ARR = [5, 3, 8,...read more
Ans. 
Implement Merge Sort algorithm to sort a given sequence of numbers in non-descending order.
• Merge Sort is a divide-and-conquer algorithm that splits the array into two halves, recursively sorts each half, and then merges the sorted halves back together.

• Time complexity of Merge Sort is O(n log n) in the worst case scenario.

• It is a stable sorting algorithm, meaning that the relative order of equal elements is preserved.

• Ex...read more
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Round duration - 60 minutes
Round difficulty - Medium

More difficult than the previous round

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.

• Store the nodes at each horizontal distance in a map, updating it as you traverse the tree.

• After traversal, extract the bottom-most nodes at each horizontal distance to get the bottom view sequence.
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Round duration - 70 minutes
Round difficulty - Medium

There were no DSA questions. My interviews were based on android app development. The interviewer presented scenarios and asked how will you solve them. No particular correct answer was expected just the best approach as per me.

Q1. How would you implement multiple RecyclerViews on a magnified screen in an Android application?
Ans. 
Implement multiple RecyclerViews on a magnified screen in an Android app by adjusting item sizes and spacing.
• Adjust item sizes and spacing based on screen magnification level

• Use nested RecyclerViews or custom layouts to display multiple lists

• Implement custom item decorators to manage spacing between RecyclerViews

• Consider using a GridLayoutManager with custom span sizes for each RecyclerView
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Q2. How would you implement various activities related to cart functionality in an e-commerce application?
Ans. 
Implementing cart functionality in an e-commerce application involves managing user interactions, updating quantities, handling discounts, and processing orders.
• Create a data structure to store cart items, quantities, and prices.

• Implement add to cart, remove from cart, and update quantity functionalities.

• Handle discounts, promotions, and coupon codes.

• Calculate total price, including taxes and shipping costs.

• Implement c...read more
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Round duration - 30 minutes
Round difficulty - Easy

HR asked questions about why Mobikwik, what's your goal, how are you a good fit, etc.

Round duration - 120 Minutes
Round difficulty - Medium

The first round was an assessment round comprised of MCQ questions of DSA, RDBMS, MySQL questions and 2 coding questions to be written in java only

Q1. 
Shortest Path in a Binary Matrix Problem Statement
Given a binary matrix of size N * M where each element is either 0 or 1, find the shortest path from a source cell to a destination cell, consisting only...read more
Ans. 
Find the shortest path in a binary matrix from a source cell to a destination cell consisting only of 1s.
• Use Breadth First Search (BFS) to explore all possible paths from the source cell to the destination cell.

• Keep track of visited cells to avoid revisiting them.

• Update the path length as you traverse through the matrix.

• Return the shortest path length found or -1 if no valid path exists.
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Q2. 
Kth Smallest Element Problem Statement
You are provided with an array of integers ARR of size N and an integer K. Your task is to find and return the K-th smallest value present in the array. All elements...read more
Ans. 
Find the K-th smallest element in an array of distinct integers.
• Sort the array and return the element at index K-1.

• Use a min-heap to find the K-th smallest element efficiently.

• Implement quickselect algorithm for optimal performance.
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Round duration - 60 Minutes
Round difficulty - Medium

The basic data structure round duration 1 hour, in this Interviewer, was very friendly and asked some casual questions. After that, he asked me to code.

Q1. 
Maximum Subarray Sum Problem Statement
Given an array of integers, determine the maximum possible sum of any contiguous subarray within the array.
Example:
Input:
array = [34, -50, 42, 14, -5, 86]
Out...read more
Ans. 
Find the maximum sum of any contiguous subarray within an array of integers.
• 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 is 137.

• Example: For array [-5, -1, -8, -9]...read more
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Round duration - 60 Minutes
Round difficulty - Hard

Advance data structure round duration 1 hour, in this Interviewer asked some basic questions on oops, puzzles and asked me to code them later.

Q1. 
Reverse Linked List Problem Statement
Given a singly linked list of integers, return the head of the reversed linked list.
Example:
Initial linked list: 1 -> 2 -> 3 -> 4 -> NULL
Reversed link...read more
Ans. 
Reverse a singly linked list of integers and return the head of the reversed linked list.
• Iterate through the linked list and reverse the pointers to point to the previous node instead of the next node.

• Use three pointers to keep track of the current, previous, and next nodes while reversing the linked list.

• Update the head of the reversed linked list as the last node encountered during the reversal process.
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Q2. 
Trapping Rain Water
You are provided with an array ARR of integers representing an elevation map, where ARR[i] denotes the elevation of the ith bar. Determine the maximum amount of rainwater that can be t...read more
Ans. 
Given an array representing an elevation map, find the maximum amount of rainwater that can be trapped between the bars.
• Iterate through the array and calculate the maximum height on the left and right of each bar.

• Calculate the trapped water by finding the minimum of the maximum heights on the left and right minus the current bar height.

• Sum up the trapped water for all bars to get the total trapped rainwater.

• Example: Fo...read more
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Round duration - 30 Minutes
Round difficulty - Medium

Vice President interview round duration 30mins, in this round VP’s of different domains taking interview to the selected candidates.

Q1. 
Buy and Sell Stock Problem Statement
Imagine you are Harshad Mehta's friend, and you have been given the stock prices of a particular company for the next 'N' days. You can perform up to two buy-and-sell ...read more
Ans. 
The task is to determine the maximum profit that can be achieved by performing up to two buy-and-sell transactions on a given set of stock prices.
• Iterate through the array of stock prices to find the maximum profit that can be achieved by buying and selling stocks.

• Keep track of the maximum profit that can be obtained by performing two transactions.

• Consider all possible combinations of buying and selling stocks to maxim...read more
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Round duration - 30 Minutes
Round difficulty - Easy

This round lasted for 30mins basic HR questions were asked

Round duration - 75 Minutes
Round difficulty - Medium

The round consisted of 2 Coding based based questions.
These question were easy for as I have already done this while preparing.

Q1. 
Flip Equivalent Binary Tree Problem
Determine whether two binary trees, given by their roots 'ROOT1' and 'ROOT2', are flip equivalent. A tree can be transformed into a flip equivalent through any number o...read more
Ans. 
The problem is to determine if two binary trees are flip equivalent after performing flip operations on one of the trees.
• Perform a depth-first search (DFS) on both trees simultaneously

• At each node, check if the values are equal and the left and right subtrees are either both null or both not null

• If the above conditions are met, recursively check the flip equivalence of the left and right subtrees

• If any of the condition...read more
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Q2. 
Split Array Into Increasing Subsequences Problem Statement
You are provided with an integer array ARR of size N sorted in ascending order. Your task is to determine if it is possible to split this array i...read more
Ans. 
The task is to determine if an integer array can be split into one or more increasing subsequences with a length of at least 3.
• Check if the array can be split into increasing subsequences by iterating through the array.

• Keep track of the current subsequence and its length while iterating.

• If the difference between the current element and the previous element is not 1, start a new subsequence.

• If the length of any subseque...read more
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Round duration - 60 Minutes
Round difficulty - Easy

The interview started a bit late as I it to be at 11:30 am but started at 12:15pm So Had to wait. Apart from these the overall experience was great and the interviewer was also kind and had a smiling face.

Q1. 
Ninja and Alternating Largest Problem Statement
Ninja is given a sequence of numbers and needs to rearrange them so that every second element is greater than its neighbors on both sides.
Example:
Input:
...read more
Ans. 
The task is to rearrange the given array such that every second element is greater than its left and right element.
• Iterate through the array and check if every second element is greater than its left and right element

• If not, swap the current element with its adjacent element to satisfy the condition

• Continue this process until the entire array satisfies the condition

• Return 1 if the array satisfies the condition, else re...read more
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Round duration - 30 Minutes
Round difficulty - Easy

10:30 PM 
Interviewer was Cool.

Round duration - 5 minutes
Round difficulty - Easy

At 10:00 am