Facebook Software Engineer Intern Interview Questions and Answers

Round duration - 90 minutes
Round difficulty - Medium

 Around 65-70 people sat for the test. 15 people were shortlisted. It consisted of the following to be done in 90 minutes.

- 28 MCQs based on core CS 

- 2 Coding questions – everyone had different and random questions. Most questions were custom logic based (easy level) including some standard questions – LCS, LIS, topological sort.

Round duration - 60 minutes
Round difficulty - Easy

Interviewer was very friendly. It started with the standard – tell me about yourself / introduce yourself type of question. Then he proceeded and asked two coding questions

Q1. 
Remove Duplicates from a Sorted Linked List
Your friend has homework to complete. Help him by removing duplicates from a sorted linked list.
You are given the 'Head' of a sorted linked list. Modify the l...read more
Ans. 
Remove duplicates from a sorted linked list without adjacent duplicates.
• Traverse the linked list while comparing current and next nodes.

• If they are equal, skip the next node by updating the current node's next pointer.

• Repeat until the end of the list is reached.
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Q2. 
Course Schedule Problem Statement
You are enrolled as a student and must complete N courses, numbered from 1 to N, to earn your degree.
Some courses have prerequisites, which means that to take course i,...read more
Ans. 
Check if it is feasible to complete all courses with prerequisites.
• Create a graph representing the prerequisites with courses as nodes and edges as prerequisites.

• Perform a topological sort on the graph to check if there is a cycle (if there is, then it's not feasible to complete all courses).

• If there is no cycle, then it is feasible to complete all courses.
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Round duration - 60 minutes
Round difficulty - Medium

This round had only 1 question. The interviewer introduced himself. And he advised me to clearly understand the problem before proceeding.

Q1. 
LCA of Binary Tree Problem Statement
You are given a binary tree of distinct integers, along with two nodes, ‘X’ and ‘Y’. Your task is to determine the Lowest Common Ancestor (LCA) of ‘X’ and ‘Y’.
The LC...read more
Ans. 
Find the Lowest Common Ancestor of two nodes in a binary tree.
• Traverse the binary tree to find the paths from the root to nodes X and Y.

• Compare the paths to find the last common node, which is the Lowest Common Ancestor.

• Implement a recursive function to efficiently find the LCA.

• Handle cases where one node is an ancestor of the other.

• Consider edge cases like when X or Y is the root node.
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Round duration - 60 minutes
Round difficulty - Medium

Pretty Good Round. Fast Paced. We covered a lot of ground.

Q1. 
Binary Search Tree Iterator Problem Statement
Design and implement a class BSTIterator to perform an in-order traversal over a Binary Search Tree (BST). Your implementation should include the following me...read more
Ans. 
Design and implement a class BSTIterator to perform in-order traversal over a Binary Search Tree (BST).
• Implement a parameterized constructor to initialize the iterator with the root of the BST.

• Implement a method 'next()' to return the next smallest element in in-order traversal.

• Implement a method 'hasNext()' to check if there exists a next smallest element in traversal.

• Ensure the output is the in-order traversal of the
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Q2. 
Maximum Consecutive Ones Problem Statement
Given a binary array 'ARR' of size 'N', your task is to determine the maximum length of a sequence consisting solely of 1’s that can be obtained by converting at...read more
Ans. 
Find the maximum length of a sequence of 1's by converting at most K zeroes into ones in a binary array.
• Iterate through the array and keep track of the current window of 1's and zeroes.

• Use a sliding window approach to find the maximum length of consecutive 1's by flipping at most K zeroes.

• Update the window based on the number of zeroes flipped and keep track of the maximum length found so far.

• Return the maximum length ...read more
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Q3. 
BFS in Graph Problem Statement
You are given an undirected and disconnected graph G(V, E) having V vertices numbered from 0 to V-1 and E edges. Your task is to print its BFS traversal starting from the 0t...read more
Ans. 
BFS traversal of an undirected and disconnected graph starting from vertex 0.
• Start BFS traversal from vertex 0 and visit all connected nodes in sorted order.

• Use a queue to keep track of nodes to visit next.

• Keep a visited array to avoid revisiting nodes.

• Print the BFS traversal path as the nodes are visited.

• Example: For input V=5, E=4 and edges 0-1, 0-2, 1-3, 2-4, the BFS traversal will be [0, 1, 2, 3, 4].
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Q4. 
Dijkstra's Shortest Path Problem Statement
Given an undirected graph with V vertices (labeled 0, 1, ..., V-1) and E edges, each edge connects two nodes X and Y with a specified weight representing the dis...read more
Ans. 
Dijkstra's algorithm is used to find the shortest path distance from a source node to all other nodes in an undirected graph.
• Implement Dijkstra's algorithm to find the shortest path distance from node 0 to all other nodes in the graph.

• Use a priority queue to efficiently select the next node with the shortest distance.

• Initialize distances to all nodes as infinity, except for the source node which is 0.

• Update distances t...read more
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Round duration - 60 minutes
Round difficulty - Hard

Bar Raised Round. Definitely felt the difficulty.

Q1. 
First Non-Repeating Character Problem Statement
You are given a string consisting of English alphabet characters. Your task is to identify and return the first character in the string that does not repeat...read more
Ans. 
Identify and return the first non-repeating character in a string, or the first character if all characters repeat.
• Iterate through the string to count the frequency of each character

• Iterate through the string again to find the first character with frequency 1

• If no such character is found, return the first character of the string
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Round duration - 150 minutes
Round difficulty - Medium

We could attempt at any time out of the 3 given days.
The round was very time constrained and we could NOT go back to the question that we already attempted.

Q1. 
Merge Two Sorted Linked Lists
Given two sorted linked lists, your task is to merge them into a single sorted linked list. Return the head of the newly combined list.
Note:
The given linked lists may or ...read more
Ans. 
Merge two sorted linked lists into a single sorted linked list.
• Create a dummy node to start the merged list

• Compare nodes from both lists and add the smaller one to the merged list

• Move the pointer of the merged list and the list with the smaller node

• Handle cases where one list is exhausted before the other

• Return the head of the merged list
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Round duration - 60 minutes
Round difficulty - Hard

Evening at 5:00 pm. It was held on Amazon Chime app
Face cam was to be kept on COMPULSARILY. You could however not see the interviewer.
Codepen type environment was also used for typing my code which could be edited by the interviewer also.
(Shared document kind of)
Interviewer was very professional and was trying to push me towards getting the most optimal solution. Each and every answer was asked a counter question as to why I made that choice

Q1. 
Missing Numbers Problem Statement
You are provided with an array called ARR, consisting of distinct positive integers. Your task is to identify all the numbers that fall within the range of the smallest a...read more
Ans. 
Identify missing numbers within the range of smallest and largest elements in an array.
• Find the smallest and largest elements in the array.

• Generate a list of numbers within the range of smallest and largest elements.

• Remove the numbers that are present in the array to get the missing numbers.

• Sort and return the missing numbers.
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Q2. 
Find Maximum Element Between Two Nodes in a BST
Given a Binary Search Tree (BST) and two integers, NODE1 and NODE2, determine the maximum element found in the path between NODE1 and NODE2.
Explanation:
...read more
Ans. 
Find the maximum element between two nodes in a Binary Search Tree (BST) path.
• Traverse the BST to find the path between NODE1 and NODE2.

• Keep track of the maximum element encountered in the path.

• Return the maximum element found, or -1 if NODE1 or NODE2 are not in the BST or no elements exist between them.
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Q3. 
String Compression Problem Statement
Implement a program that performs basic string compression. When a character is consecutively repeated more than once, replace the consecutive duplicates with the coun...read more
Ans. 
Implement a program to compress a string by replacing consecutive duplicates with the count of repetitions.
• Iterate through the string and keep track of consecutive characters and their counts.

• Replace consecutive duplicates with the count of repetitions.

• Ensure the count of repetitions is ≤ 9.

• Return the compressed string.
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Round duration - 90 minutes
Round difficulty - Medium

Q1. 
Count Inversions Problem Statement
Given an integer array ARR of size N, your task is to find the total number of inversions that exist in the array.
An inversion is defined for a pair of integers in the...read more
Ans. 
Count the total number of inversions in an integer array.
• Iterate through the array and for each pair of elements, check if the conditions for inversion are met.

• Use a nested loop to compare each element with all elements to its right.

• Keep a count of the inversions found and return the total count at the end.
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Q2. 
Postfix Expression Evaluation Problem Statement
Given a postfix expression, your task is to evaluate the expression. The operator will appear in the expression after the operands. The output for each expr...read more
Ans. 
Evaluate postfix expressions by applying operators after operands. Return result modulo (10^9+7).
• Iterate through each character in the postfix expression

• If character is an operand, push it onto the stack

• If character is an operator, pop two operands from stack, apply operator, and push result back

• Continue until end of expression, return final result modulo (10^9+7)
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Round duration - 40 minutes
Round difficulty - Medium

Q1. 
String Rearrangement Problem Statement
You are given a string of lowercase characters. The objective is to rearrange (reorder) the string so that no two adjacent characters are identical.
Return the rear...read more
Ans. 
Given a string of lowercase characters, rearrange it so that no two adjacent characters are identical. Return the rearranged string or 'NO SOLUTION'.
• Iterate through the string and count the frequency of each character.

• Sort the characters based on their frequency in non-increasing order.

• Place the characters with highest frequency first, alternating with other characters.

• If there are more than half of the characters with
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Q2. 
Pair Sum Problem Statement
Given an integer array ARR of size N and an integer S, your task is to return a list of all pairs of elements such that the sum of each pair equals S.
Input:
The first line co...read more
Ans. 
The task is to find pairs of elements in an array that sum up to a given target value.
• Iterate through the array and for each element, check if the complement (target - current element) exists in a hash set.

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

• Ensure pairs are sorted based on the criteria mentioned in the question.
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Round duration - 40 minutes
Round difficulty - Medium

Q1. 
Find Number of Islands
You are provided with a 2-dimensional array/list of size N rows by M columns, containing ones (1) and zeroes (0). Here, 1 represents land, and 0 represents water.
A cell is consider...read more
Ans. 
Find the number of islands in a 2D matrix of 1s and 0s.
• Iterate through the matrix and perform depth-first search (DFS) to find connected 1s.

• Use a visited array to keep track of visited cells to avoid counting the same island multiple times.

• Increment the island count whenever a new island is encountered.
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Q2. 
Triplets in a Sorted Doubly Linked List
Given a sorted doubly linked list of distinct nodes, determine how many triplets in the list have a sum equal to a given value 'X'. Each node in the list contains a...read more
Ans. 
Given a sorted doubly linked list of distinct nodes, find and count triplets with a sum equal to a given value 'X'.
• Iterate through the list and for each node, use two pointers to find the other two nodes that sum up to 'X'.

• Keep track of the count of triplets found and return it at the end.

• Handle edge cases like when the list has less than 3 nodes or no triplets are found.
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Round duration - 60 minutes
Round difficulty - Medium

In this round i was tested for Data Structure competency. Two questions were asked.

Q1. 
Group Anagrams Problem Statement
Given an array or list of strings called inputStr, your task is to return the strings grouped as anagrams. Each group should contain strings that are anagrams of one anoth...read more
Ans. 
Group anagrams in an array of strings based on their characters.
• Iterate through the array of strings and sort each string to group anagrams together.

• Use a hashmap to store the sorted string as key and the list of anagrams as value.

• Return the values of the hashmap as the grouped anagrams.
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Round duration - 90 minutes
Round difficulty - Medium

Two questions were asked in this round on data structures and algorithms.

Q1. 
BST Iterator Problem Statement
You are tasked with creating a class named BSTIterator that acts as an iterator for the inorder traversal of a binary search tree. Implement the following functions:
1. BST...read more
Ans. 
Implement a BSTIterator class for inorder traversal of a binary search tree.
• Create a BSTIterator class with constructor, next, and hasNext functions.

• Use a stack to simulate inorder traversal of the binary search tree.

• Ensure the hasNext function returns true if there are more elements to traverse.

• Example: Input - 4 2 6 1 3 5 7 -1 -1 -1 -1 -1 -1, Output - 1 2 3 4 5 6 7
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Q2. 
K Most Frequent Words Problem Statement
Given an array or list WORDS of 'N' non-empty words and an integer 'K', identify the 'K' most frequent words and return them sorted by their frequency, in descendin...read more
Ans. 
Identify the K most frequent words in a list, sorted by frequency and lexicographical order.
• Use a hashmap to store word frequencies

• Use a min heap to keep track of K most frequent words

• Sort the heap based on frequency and lexicographical order
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Round duration - 90 minutes
Round difficulty - Medium

Q1. 
String Transformation Problem Statement
Given a string str of length N, perform a series of operations to create a new string:
1. Select the smallest character from the first 'K' characters of the string,...read more
Ans. 
The problem involves selecting the smallest character from the first 'K' characters of a string and appending it to a new string until the original string is empty.
• Iterate through the string, selecting the smallest character from the first 'K' characters each time.

• Remove the selected character from the original string and append it to the new string.

• If fewer than 'K' characters remain, sort and append them in order.

• Rep...read more
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Q2. 
Counting Derangements Problem
A derangement is a permutation of 'N' elements, where no element appears in its original position. For instance, a derangement of {0, 1, 2, 3} is {2, 3, 1, 0} because each el...read more
Ans. 
Count the total number of derangements possible for a set of 'N' elements.
• A derangement is a permutation where no element appears in its original position.

• Use the formula for derangements: !n = n! * (1 - 1/1! + 1/2! - 1/3! + ... + (-1)^n/n!).

• Return the answer modulo (10^9 + 7) as the result could be very large.
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Round duration - 60 minutes
Round difficulty - Medium

1 hour online interview. 
Slot was given prior via email.
The HR contacted me via email and simultaneously provided me a link where I had to write the codes during my interview

Q1. 
Mike and Mobile Keypad Problem
Mike, a young math enthusiast, is playing with his mom’s mobile phone, which has a keypad with 12 buttons (10 digits: 0-9, and 2 special characters: ‘*’ and ‘#’). His challe...read more
Ans. 
Calculate the number of distinct numbers that can be formed by pressing 'N' buttons on a mobile keypad following specific rules.
• Use dynamic programming to keep track of the number of distinct numbers that can be formed at each step.

• Consider the possible transitions from each button to calculate the total number of distinct numbers.

• Implement a solution that efficiently handles large values by using modulo arithmetic.

• Ens...read more
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Q2. 
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.

• Use Kadane's algorithm to efficiently find the maximum subarray sum in O(N) time complexity.
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Q3. 
Maximum Sum of Non-Adjacent Elements
Given an array/list of ‘N’ integers, your task is to return the maximum sum of the subsequence where no two elements are adjacent in the given array/list.
Example:
I...read more
Ans. 
Find the maximum sum of non-adjacent elements in an array.
• Use dynamic programming to keep track of the maximum sum at each index, considering whether to include the current element or skip it.

• At each index, the maximum sum is either the sum of the current element and the element two positions back, or the sum at the previous index.

• Iterate through the array and update the maximum sum at each index accordingly.

• Return the...read more
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Q4. 
Trapping Rainwater Problem Statement
You are given an array ARR of long type, which represents an elevation map where ARR[i] denotes the elevation of the ith bar. Calculate the total amount of rainwater t...read more
Ans. 
Calculate the total amount of rainwater that can be trapped within given elevation map.
• Iterate through the array to find the maximum height on the left and right of each bar.

• Calculate the amount of water that can be trapped above each bar by taking the minimum of the maximum heights on the left and right.

• Sum up the trapped water above each bar to get the total trapped water for the elevation map.
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Round duration - 60 minutes
Round difficulty - Medium

Q1. 
Strongly Connected Components (Tarjan’s Algorithm) Problem Statement
Given an unweighted directed graph with V vertices and E edges, your task is to identify and print all the strongly connected component...read more
Ans. 
Tarjan's Algorithm is used to find strongly connected components in an unweighted directed graph.
• Tarjan's Algorithm is a depth-first search based algorithm.

• It uses a stack to keep track of vertices in the current SCC.

• The algorithm assigns a unique id to each vertex and keeps track of the lowest id reachable from the current vertex.

• Once a SCC is identified, the vertices in the stack form that SCC.

• The algorithm runs in O...read more
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Q2. 
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 achie...read more
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Round duration - 60 minutes
Round difficulty - Easy

This was a one hour round in which I was first told to introduce myself and then moved to questions.

Q1. 
Permutation in String Problem Statement
Determine if the string str2 contains a permutation of the string str1 as one of its substrings.
Input:
The first line contains an integer 'T', representing the n...read more
Ans. 
Check if a permutation of one string is a substring of another string.
• Create a frequency map of characters in str1.

• Iterate through str2 with a window of size N and check if the frequency map matches.

• Return 'Yes' if a permutation is found, 'No' otherwise.
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Q2. 
Sort 0s, 1s, and 2s Problem Statement
You are provided with an integer array/list ARR of size 'N' which consists solely of 0s, 1s, and 2s. Your task is to write a solution to sort this array/list.
Input:
...read more
Ans. 
Sort an array of 0s, 1s, and 2s in linear time with a single scan approach.
• Use three pointers to keep track of the positions of 0s, 1s, and 2s in the array.

• Iterate through the array once and swap elements based on their values and the pointers.

• After the single scan, the array will be sorted in the required order.
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Round duration - 60 minutes
Round difficulty - Medium

This was based more on my communication skills rather than my technical skills. The interviewer didn't only focus on whether I knew the answer or not but if I was able to communicate my knowledge to the interviewer.

It was more of theory based interview
Questions are as follows:-
1. What are SQL and acid properties?

2. How does a website start?

3. What are deadlocks with real life examples? Give necessary conditions for deadlock. 
-> Real life examples were must. 

4. What is virtual function?

5. Tell me the most challenging this while you made your project. ( I had to pick my project on my own) 

6. Tell me the most interesting feature you found working on a language. 

7. Technical question on structures in C as I used it in my project. The interviewer asked if the structure for a linked list is changed as:-
struct node

{
int data;
struct next; 
};
I explained how this won't work as struct will be created again and again which will lead to memory limit exceeded.

Q1. 
Delete Alternate Nodes from a Singly Linked List
Given a Singly Linked List of integers, remove all the alternate nodes from the list.
Input:
The first and the only line of input will contain the elemen...read more
Ans. 
Remove alternate nodes from a singly linked list of integers.
• Traverse the linked list and skip every alternate node while updating the next pointers.

• Make sure to handle cases where there are less than 3 nodes in the list.

• Update the next pointers accordingly to remove alternate nodes.

• Example: Input: 10 20 30 40 50 60 -1, Output: 10 30 50
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