Paytm Full Stack Developer Interview Questions, Process, and Tips

Round duration - 70 minutes
Round difficulty - Medium

There were 3 problems 1 was easy and 2 were of medium types.

Q1. Odd even level
Given a binary tree. Find and return the modulus of the difference between the sum of odd level nodes and the sum of even level nodes.
Input format:
The first line contains an integer '...read more
Ans. DFS
1. Create two variables, one for storing the sum of nodes at the odd level and one for storing the sum of nodes at even level.
2. Use the below algorithm to calculate level sums where:
oddSum: denotes sum of nodes at odd levels, initially 0.
evenSum: denotes sum of nodes at ecen levels, initially 0.
level: the level of the current node, initially 1. 
void oddEvenLevelHelper(current, oddSum, evenSum, level)
 
• If the cur...read more
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Q2. Rotting Oranges
You have been given a grid containing some oranges. Each cell of this grid has one of the three integers values:
Value 0 - representing an empty cell.
Value 1 - representing a fresh...read more
Ans. Naïve Solution
The idea is very simple and naive. We will process the rotten oranges second by second. Each second, we rot all the fresh oranges that are adjacent to the already rotten oranges. The time by which there are no rotten oranges left to process will be our minimum time.
 
In the first traversal of the grid, we will process all the cells with value 2 (rotten oranges). We will also mark their adjacent cells ...read more
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Round duration - 50 minutes
Round difficulty - Medium

the interviewer was very polite and straightforward, he didn't ask me to introduce myself and he directly jumps to the coding problems.
What is new about the relationship between the and tags in HTML5?
Explain Components, Modules and Services in Angular

Q1. Circular Move
You have a robot currently standing at the origin (0, 0) of a two-dimensional grid and facing north direction. You are given a sequence of moves for the robot in the form of a string of size...read more
Ans. Optimal Approach
Initialize a variable ‘direction’ with 0 which means that the robot is initially facing towards the north.
direction: 0 -> Robot is facing towards the North
direction: 1 -> Robot is facing towards the West 
direction: 2 -> Robot is facing towards the South 
direction: 3 -> Robot is facing towards the West
 
Initialize two variables ‘x’ and ‘y’ as 0. They will represent the position ...read more
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Round duration - 70 minutes
Round difficulty - Easy

What is a View in sql? What are the TRUNCATE, DELETE and DROP statements?

Q1. Rotate matrix by 90 degrees
You are given a square matrix of non-negative integers 'MATRIX'. Your task is to rotate that array by 90 degrees in an anti-clockwise direction using constant extra spac...read more
Ans. Transpose and Reverse
The idea is to find the transpose of the given matrix and then reverse the columns of the transposed matrix. For example:
For the given 2D matrix:
	[ [ 1,  2,  3 ],
	  [ 4,  5,  6 ],
	  [ 7,  8,  9 ] ]
After taking transpose, it will become:
	[ [ 1,  4,  7 ],
	  [ 2,  5,  8 ],
	  [ 3,  6,  9 ] ]
After reversing the columns, it will ...read more
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Q2. Minimum Characters For Palindrome
Given a string STR of length N. The task is to return the count of minimum characters to be added at front to make the string a palindrome.
For example, for the given st...read more
Ans. Brute Force
• The idea is pretty simple, as we can add only at the front, thus, we need to work on the prefix of the string.
• We need to find the largest prefix that is a palindrome. For example, in case of “abbac”, the largest prefix that is a palindrome is “abba”, now all you need to do is add the reverse of left out suffix i.e. “ca” at the front to make it a palindrome.
• This can be done in the following way:-
  • Until the str...read more
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Round duration - 20 minutes
Round difficulty - Easy

Q1. Basic HR Questions
Tell me about yourself?
What do you know about Paytm?
What are your strength and weaknesses?
Job location preference?
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Round duration - 70 minutes
Round difficulty - Medium

The test started around 3 pm and was of 70 minutes. 3 coding questions were asked which were mainly on trees and number theory.

Q1. Factorial of a Number
Write a program to find the factorial of a number.
Factorial of n is:
n! = n * (n-1) * (n-2) * (n-3)....* 1
Output the factorial of 'n'. If it does not exist, output ...read more
Ans. Using while() loop
• This can be done using an if-else-if ladder to check for negative,0, and positive numbers.
• If the input is a positive number use a while loop to multiply n with n-1, n-2…1 and get the desired output.
Space Complexity: O(1)Explanation: Time Complexity: O(1)Explanation:
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Q2. Next Greater Element
For a given array/list of integers of size N, print the Next Greater Element(NGE) for every element. The Next Greater Element for an element X is the first element on the right side of...read more
Ans. Brute Force
For every element in the array, we will run a loop on its right side. As soon as we find an element on its right side which is greater than it, we will break the loop, assign it as the NGE of this element, move forward, and do the same for the next element.
Space Complexity: O(1)Explanation:
O(1)
 
No extra space is used.
Time Complexity: O(n^2)Explanation:
O(N ^ 2),  Where N is the number of elements ...read more
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Q3. Ceil from BST
Ninja is given a binary search tree and an integer. Now he is given a particular key in the tree and returns its ceil value. Can you help Ninja solve the problem?
Note:
Ceil of an integer i...read more
Ans. Brute Force
We will traverse the Binary search tree from the root node till we find the node whose key value is given, and upon getting that, we return the ceil value of it.

 
The steps are as follows:  
• Initialize ‘ceilValue’ to store the ceil value to be returned.
• We run a while loop till the value of the node is not equal to NULL.
  • If the value of the node is equal to ‘X’’ then we return the node value.
  • If th...read more
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Round duration - 60 minutes
Round difficulty - Medium

Interviewer was very friendly and supportive. My interview happened on google meet. What is a View? What is Denormalization? Difference between “==” and “===” operators in js.

Q1. Sub Sort
You are given an integer array ‘ARR’. You have to find the length of the shortest contiguous subarray such that, if you sort this subarray in ascending order, then the whole array will be sorted i...read more
Ans. Brute Force
In this approach, we consider every possible subarray that can be formed from the given array ‘ARR’. For every subarray ARR[i … j] considered, we need to check whether this is the smallest unsorted subarray or not.
 
The algorithm is as follows:
1. Let’s take the variable ‘ANS’ equal to 'N' (the length of the array) initially.
2. Now for every subarray ARR[i … j], we will find the maximum and minimum value lying ...read more
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Q2. Sort 0 1 2
You have been given an integer array/list(ARR) of size 'N'. It only contains 0s, 1s and 2s. Write a solution to sort this array/list.
Note :
Try to solve the problem in 'Single Sca...read more
Ans. Sorting
Use any good sorting algorithm like Merge Sort, Quick Sort or inbuilt sorting function of different languages.
• Sort the Array and just return.
Space Complexity: O(1)Explanation:
O(1), As we are using constant space.
Time Complexity: O(nlogn)Explanation:
O(N*log(N)), where ‘N’ is the size of the array.
We are using inbuilt sort algorithm which has Overall Time Complexity O(N*log(N))
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Round duration - 60 minutes
Round difficulty - Medium

What are some of the advantages of Angular over other frameworks?

Q1. 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, you need to check if these no...read more
Ans. Recursive Solution
• Find the levels of both the given nodes.
• This can be done by making a separate function for finding the level/by using level order traversal.
• The levels should be the same.
• Check if both the nodes are siblings or not.
• This can again be done by simply going through the entire Binary Tree and checking if the node’s children are the given two nodes. If this is true, then given two nodes are siblings and henc...read more
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Q2. OS Questions
What is spooling?
What are the conditions for deadlock to occur?
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Round duration - 10 minutes
Round difficulty - Easy

I got a call from HR in the evening around 6:30 pm.

Q1. Basic HR Question
Tell me about yourself?
Ans. 
Tip 1 : Do prepare such questions before hand only.
Tip 2 : Speak confidently.
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Q2. Basic HR Question
How was your experience with previous two tech rounds?
Ans. 
Tip 1 : Do remember the names of the interviewers who took your interview.
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Q3. Basic HR Question
What do you know about Paytm?
Ans. 
Tip 1 : Do a research before hand about what does the company do.
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Round duration - 70 minutes
Round difficulty - Easy

there was some coding questions and some mcqs related to js, html, css, java and aptitude.

Q1. Bridges In A Graph
Given an undirected graph of V vertices and E edges. Your task is to find all the bridges in the given undirected graph. A bridge in any graph is defined as an edge which, when removed, ...read more
Ans. Brute Force
The brute force approach to find all the bridges in a given graph is to check for every edge if it is a bridge or not, by first not considering current edge is not in given graph and then checking if the vertices that it was connecting are still connected or not, using DFS(DepthFirst Search). Given below is an algorithm that will find all bridges in a graph.
 
Algorithm :
1. Initialise a visited array with fa...read more
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Q2. House Robber
Mr. X is a professional robber planning to rob houses along a street. Each house has a certain amount of money hidden. All houses along this street are arranged in a circle. That means the fir...read more
Ans. 
The task is to find the maximum amount of money Mr. X can rob from houses arranged in a circle without alerting the police.
• The problem can be solved using dynamic programming.

• Create two arrays to store the maximum amount of money robbed when considering the first house and when not considering the first house.

• Iterate through the array and update the maximum amount of money robbed at each house.

• The final answer will be
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Round duration - 90 minutes
Round difficulty - Medium

It was in the evening, at around 3 - 4 PM. There were 3 coding questions. One easy, two moderate ones. The platform had autocomplete feature too.

Q1. Parity Move
Ninja recently studied odd and even numbers but he is more interested in even numbers.
He has an array ‘A’ containing ‘N’ integers. He will transform this array by moving all the even numbers...read more
Ans. Sorting Approach
 
Approach : 
 
• Implement a comparator that check numbers ‘a’ and ‘b’ :
• If both are even or both are odd, it can return any priority between them as we need to sort by parity here.
• If one of them is even and the other is odd, prioritize even numbers to order them first.

 
Algorithm : 
 
• Call the comparator function on array ‘A’ that prioritizes the least among two integers ‘a%2’ and...read more
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Q2. Anagram Pairs
Pre-requisites: Anagrams are defined as words or names that can be formed by rearranging letters of another word. Such as "spar" can be formed by rearranging letters of "rasp&q...read more
Ans. Count Characters
Anagrams have a unique property: the counts of each distinct character present in both strings are the same. One way to check this is: 
1. Sort both strings, so that all the same characters come together
2. Then loop through both strings together and check each element in both strings one by one
3. If at any position, the characters are found to be different or if the lengths of the two strings are different, ...read more
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Q3. LCA In A BST
You are given a binary search tree of integers with N nodes. You are also given references to two nodes P and Q from this BST.
Your task is to find the lowest common ancestor(LCA) of these t...read more
Ans. Depth - First Traversal
We will traverse the BST in a depth-first manner. The moment we encounter either of the nodes P or Q, we will return some boolean flag. The least common ancestor would then be the node for which both the subtree recursions return a ‘True’  flag value.   
 
The algorithm will be -
• We will start traversing a BST from the root node in a recursive manner.
• Let the current node in each...read more
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Round duration - 50 Minutes
Round difficulty - Medium

They asked 2 coding questions and 2 sql queries in this round

Q1. Longest Palindromic Substring
Given a string ’S’ consisting of lower case English letters, you are supposed to return the longest palindromic substring of ‘S’.
Note that in case of more than one longest ...read more
Ans. Brute Force
The brute force solution is to pick all possible starting and ending positions for a substring and verify if it is a palindrome.
 
The steps are as follows:
 
1. If the length of the given string is zero, return an empty string. Else,
2. Start checking each substring of the given string ‘S’ whether the substring is a palindrome or not.
3. To do this first, run the two nested loops.
4. These two loops pick up all the ...read more
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Q2. Trapping Rain Water
You have been given a long type array/list 'ARR' of size 'N'. It represents an elevation map wherein 'ARR[i]' denotes the elevation of the 'ith' bar. Pri...read more
Ans. Brute Force Approach
The idea here is to travel over every elevation on the map and calculate the units of water the elevation can store.
 
Here is the algorithm :
 
1. Iterate over every elevation or element and find the maximum elevation on to the left and right of it. Say, the maximum elevation on to the left of the current elevation or element that we are looking at is ‘maxLeftHeight’ and the maximum elevation on...read more
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Q3. SQL Question
A nested SQL query
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Round duration - 50 Minutes
Round difficulty - Medium

This round was about OS, networking and my projects. What are the different ways to style a React component?
Name a few techniques to optimize React app performance.

Q1. OS Questions
Theory questions about OS . Multithreading, Scheduling Algorithms
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Q2. Puzzle
There are 12 coins. 11 of them have same weight and one is light in weight than others. We have a weighing balance. Find the minimum times we have to balance to find out the lighter one.
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Round duration - 30 Minutes
Round difficulty - Easy

It was a fun round, in which they ask about your preferences about programming languages, technologies or your hobbies. If you reach this round, it means that your chances of selection are 99%.

Q1. Basic HR Question
What was the biggest hurdle you faced while developing a project
Ans. 
Tip 1 : Be truthful, do build some projects on your own so that you can answer these kind of questions truthfully.
Tip 2 : Be confident
Tip 3 : Ask any questions you want to ask from the interviewer
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