Facebook Software Developer Intern Interview Questions, Process, and Tips

Round duration - 60 minutes
Round difficulty - Easy

There was 2 coding questions
Based on ds and algorithms

Q1. 
Ninja's Pattern with Powers of 2
Ninja, who loves playing with numbers, sets out to arrange numbers within 'N' rows. The unique arrangement follows these rules: the first row contains 1 number, the second...read more
Ans. 
Generate a pattern of numbers in rows following a specific sequence based on powers of 2.
• Start with 1 number in the first row, 2 numbers in the second row, 4 numbers in the third row, and so on based on powers of 2.

• Fill the pattern with numbers in increasing sequence from 1 to 9, recycling back to 1 after reaching 9.

• Output the pattern for a given number 'N' of rows.

• Example: For N = 4, the pattern would be 1, 23, 4567,
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Q2. 
Validate Binary Search Tree (BST)
You are given a binary tree with 'N' integer nodes. Your task is to determine whether this binary tree is a Binary Search Tree (BST).
BST Definition:
A Binary Search Tr...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 data less than the node's data.

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

• Recursively check if both the left and right subtrees are also binary search trees.

• Example: For a node with data 4, left subtree nodes (2, 1, 3) are smaller and righ
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Round duration - 60 minutes
Round difficulty - Easy

There was 2 coding questions based on ds and algorithms

Q1. 
Zig-Zag Array Rearrangement
You are provided with an array of distinct elements, and your task is to rearrange the array elements in a zig-zag manner. Specifically, for every odd index i, the element ARR[...read more
Ans. 
Rearrange array elements in a zig-zag manner where every odd index element is greater than its neighbors.
• Iterate through the array and swap elements to satisfy the zig-zag condition.

• Ensure that for every odd index i, ARR[i] > ARR[i-1] and ARR[i] > ARR[i+1].

• Multiple correct answers may exist for a given array.
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Q2. 
Flatten a Linked List
You are provided with a linked list of 'N' nodes. Each node contains two pointers: NEXT, pointing to the next node in the list, and CHILD, pointing to a linked list. Each child linke...read more
Ans. 
Flatten a linked list of sorted child linked lists into a single sorted linked list.
• Traverse the linked list and maintain a priority queue to keep track of the next smallest element.

• Merge the child linked lists into the priority queue while traversing the main linked list.

• Pop elements from the priority queue to create the final flattened linked list.
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Round duration - 45 Minutes
Round difficulty - Medium

My interview started at 9:30 and it took around 45 minutes to complete my interview.This was held on Amazon Chime and the interview lasted for 1 hour. Firstly the interviewer asked to introduce about myself, later asked regarding the projects I have mentioned in my resume. Then started displaying the coding question. The first question is number of islands in a matrix.

Q1. 
Number of Islands Problem Statement
You are provided with a 2-dimensional matrix having N rows and M columns, containing only 1s (land) and 0s (water). Your goal is to determine the number of islands in t...read more
Ans. 
Count the number of islands in a 2D matrix of 1s and 0s.
• Use Depth First Search (DFS) or Breadth First Search (BFS) to traverse the matrix and identify connected groups of 1s.

• Maintain a visited array to keep track of visited cells to avoid redundant traversal.

• Increment the island count each time a new island is encountered.

• Consider edge cases like when the matrix is empty or all cells are water (0s).
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Round duration - 90 minutes
Round difficulty - Easy

This was an online technical round on the mettl platform, the test window was open from 22 to 25 May 2020
The test had 28 mcqs and 2 coding questions.
The test was proctored. One needed to have webcam on.
A sample test link was provided before test to get familiar with the mettl platform.

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 pair of elements efficiently.

• Keep a count of the inversions found and return the total count at the end.
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Q2. 
Find Pair with Maximum GCD in an Array
Given an array of positive integers, your task is to find the GCD (Greatest Common Divisor) of a pair of elements such that it is the maximum among all possible pair...read more
Ans. 
Find the pair with the maximum GCD in an array of positive integers.
• Iterate through all pairs of elements in the array.

• Calculate the GCD of each pair using Euclidean algorithm.

• Keep track of the maximum GCD found so far.

• Return the maximum GCD value.
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Round duration - 60 minutes
Round difficulty - Medium

Time : 12pm to 1pm
Mode of Interview : Amazon Chime Video
LiveCode : To write code, it was not a compiler
The interviewer was quite supportive.

Q1. 
Majority Element Problem Statement
Given an array/list 'ARR' consisting of 'N' integers, your task is to find the majority element in the array. If there is no majority element present, return -1.
Exampl...read more
Ans. 
Find the majority element in an array, return -1 if no majority element exists.
• Iterate through the array and keep track of the count of each element using a hashmap.

• Check if any element's count is greater than floor(N/2) to determine the majority element.

• Return the majority element or -1 if no majority element exists.
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Q2. 
Subarray with Equal Occurrences Problem Statement
You are provided with an array/list ARR of length N containing only 0s and 1s. Your goal is to determine the number of non-empty subarrays where the numbe...read more
Ans. 
Count the number of subarrays where the number of 0s is equal to the number of 1s in a given array of 0s and 1s.
• Iterate through the array and keep track of the count of 0s and 1s encountered so far.

• Use a hashmap to store the difference between the counts of 0s and 1s seen at each index.

• Increment the count of subarrays whenever the difference between the counts seen before matches the current difference.
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Round duration - 60 minutes
Round difficulty - Medium

Time : 11 am to 12am
Mode of Interview : Amazon Chime Video
LiveCode : To write code, it was not a compiler
The interviewer was quite supportive.

Q1. 
Sorted Linked List to Balanced BST Problem Statement
Given a singly linked list where nodes contain values in increasing order, your task is to convert it into a Balanced Binary Search Tree (BST) using th...read more
Ans. 
Convert a sorted linked list into a Balanced Binary Search Tree (BST) using the same data values.
• Create a function to convert the linked list to an array for easier manipulation.

• Implement a function to build a Balanced BST from the array recursively.

• Ensure the height difference of the subtrees is no more than 1 for each node.

• Use level order traversal to output the values of the BST nodes.

• Handle NULL nodes by representi
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Q2. 
Implement Stack with Linked List
Your task is to implement a Stack data structure using a Singly Linked List.
Explanation:
Create a class named Stack which supports the following operations, each in O(1...read more
Ans. 
Implement a Stack data structure using a Singly Linked List with operations in O(1) time.
• Create a class named Stack with getSize, isEmpty, push, pop, and getTop methods.

• Use a Singly Linked List to store the elements of the stack.

• Ensure each operation runs in O(1) time complexity.

• Handle edge cases like empty stack appropriately.

• Test the implementation with sample queries to verify correctness.
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Round duration - 215 minutes
Round difficulty - Medium

* Debugging(C, C++, Java, Python) [Time given was quite enough.]
* Coding(Any programming language was allowed. 2 coding questions medium to difficult level 70 mins)
* Psychometric test based on Leadership principle — Don’t take it for granted. I would suggest to pay attention while 
answering to this as well.
* Aptitude and Logical reasoning (Good enough time was given.)

Q1. 
Weird Number Pattern Problem Statement
You are given an integer 'N'. Your task is to print a specific pattern for the given number of rows 'N'.
Input:
The first line contains a single integer ‘T’ repres...read more
Ans. 
Print a specific pattern of '1's for a given number of rows 'N'.
• Read the number of test cases 'T'

• For each test case, read the number of rows 'N'

• Print '1' repeated i times for each line i from 1 to N
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Q2. 
Find All Pairs Adding Up to Target
Given an array of integers ARR of length N and an integer Target, your task is to return all pairs of elements such that they add up to the Target.
Input:
The first line ...read more
Ans. 
Given an array of integers and a target, find all pairs of elements that add up to the target.
• 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. Otherwise, add the current element to the hash set.

• Handle cases where the same element is used twice in a pair.

• Return (-1, -1) if no pair is found.
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Round duration - 60 minutes
Round difficulty - Medium

(Time — 5.30 p.m.): I introduced myself. My interviewer told me that he would divide the interview into three phases.
Phase 1- Introduction and questions on life instances where I proved my efficiency. (barely 10 minutes, I answered well.)
Phase 2- Data Structures’ Understanding and questions related to it. (Hashing concept and complete conceptual details about it. Internal working, collisions, how to avoid them etc.) (15 minutes, He was satisfied with my answers. Don’t just answer. Explain !)

Q1. 
Longest Unique Substring Problem Statement
Given a string Str consisting of N lowercase Latin letters, you need to determine the longest substring without repeating characters.
A substring is defined as ...read more
Ans. 
Given a string, find the longest substring without repeating characters.
• Use a sliding window approach to keep track of the longest substring without repeating characters.

• Use a hashmap to store the index of each character as it appears in the string.

• Update the start index of the window when a repeating character is found.

• Keep track of the longest substring length and its starting index.

• Return the substring starting from...read more
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Round duration - 70 minutes
Round difficulty - Hard

(Time — 7.00 p.m.): I introduced myself. The interviewer directly jumped onto the questions. He asked me two questions. The first question he asked me was: Coding Question

Q1. 
Longest Substring with At Most K Distinct Characters
Given a string S of length N and an integer K, find the length of the longest substring that contains at most K distinct characters.
Input:
The first...read more
Ans. 
Find the length of the longest substring with at most K distinct characters in a given string.
• Use a sliding window approach to keep track of the characters and their counts within the window.

• Maintain a hashmap to store the characters and their frequencies.

• Update the window size and characters count as you iterate through the string.

• Return the maximum window size encountered for each test case.
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Q2. 
Count Set Bits Problem Statement
Given a positive integer N, compute the total number of '1's in the binary representation of all numbers from 1 to N. Return this count modulo 1e9+7 because the result can...read more
Ans. 
Count the total number of set bits in the binary representation of numbers from 1 to N modulo 1e9+7.
• Use bitwise operations to count the set bits in each number from 1 to N.

• Consider using dynamic programming to optimize the solution.

• Remember to return the count modulo 1e9+7 to handle large results.
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Round duration - 2 hour 30 mins
Round difficulty - Easy

Debugging: This section had 7 debugging problems, which consist of code snippets that have some logical error that needs to be rectified.
Reasoning Ability: This section consists of some verbal reasoning questions and some aptitude questions.
Coding: This section consists of 2 coding problems.

Q1. 
Search in a Row-wise and Column-wise Sorted Matrix Problem Statement
You are given an N * N matrix of integers where each row and each column is sorted in increasing order. Your task is to find the positi...read more
Ans. 
Given a sorted N * N matrix, find the position of a target integer X within the matrix.
• Iterate over each row and column to search for the target integer X

• Utilize the sorted nature of the matrix to optimize the search process

• Return the position of X if found, else return -1 -1
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Q2. 
Clone Linked List with Random Pointer Problem Statement
Given a linked list where each node has two pointers: one pointing to the next node and another which can point randomly to any node in the list or ...read more
Ans. 
Cloning a linked list with random pointers by creating new nodes rather than copying references.
• Create a deep copy of the linked list by iterating through the original list and creating new nodes with the same values.

• Update the random pointers of the new nodes by mapping the original node's random pointer index to the corresponding new node.

• Ensure the cloned linked list is an exact copy of the original by validating th...read more
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Round duration - 60 minutes
Round difficulty - Medium

Around 30 candidates were shortlisted from my campus and over 150+ candidates were shortlisted from the university, and I was one of them. Then my first round of interviews was scheduled. I was pretty nervous before the interview.

Q1. 
Return in Row Wave Form Problem Statement
You are provided with a 2D array having dimensions 'N*M'. Your task is to traverse the elements row-wise and return a single-dimensional array which stores these ...read more
Ans. 
Traverse a 2D array row-wise and return elements in a wave pattern.
• Traverse the 2D array row-wise and store elements alternately in a wave pattern.

• For each row, store elements from left to right for odd rows and from right to left for even rows.

• Return the final 1D array representing the wave pattern of elements.
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Q2. 
Strict Binary Tree Construction
Given two lists, one representing the preorder traversal ('PRE') of a strict binary tree and the other ('TYPENL') indicating if each node is a leaf ('L') or non-leaf ('N')....read more
Ans. 
Construct a strict binary tree from preorder traversal and leaf/non-leaf indicators.
• Create a binary tree node class with value and left/right pointers.

• Use a stack to keep track of parent nodes while constructing the tree.

• Check if the current node is a leaf or non-leaf based on 'TYPENL' list.
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Round duration - 60 minutes
Round difficulty - Medium

My interviewer introduced himself in the beginning and asked for my introduction.

Q1. 
Alien Dictionary Problem Statement
You've been provided with a sorted dictionary in an alien language. Your task is to determine the character order of this alien language from this dictionary. The dictio...read more
Ans. 
Given a sorted dictionary in an alien language, determine the character order of the language.
• Iterate through the dictionary to find the order of characters based on their appearance in words.

• Create a graph of characters and their relationships based on adjacent words in the dictionary.

• Perform a topological sort on the graph to determine the character order.

• Return the list of characters in the correct order as the outp
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Round duration - 45 minutes
Round difficulty - Medium

The first Round was held on Hackerrank and the questions were of medium difficulty based on Data Structures and Algorithms.
The time of test was 1:00 PM and it was of 45 minutes with 2 coding questions to be solved.

Q1. 
Merge Overlapping Intervals Problem Statement
Given a specified number of intervals, where each interval is represented by two integers denoting its boundaries, the task is to merge all overlapping interv...read more
Ans. 
Merge overlapping intervals and return sorted list of merged intervals.
• Identify overlapping intervals based on start and end times

• Merge overlapping intervals to form new intervals

• Sort the merged intervals in ascending order of start times
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Q2. 
Josephus Problem Statement
Consider 'N' individuals standing in a circle, numbered consecutively from 1 to N, in a clockwise direction. Initially, the person at position 1 starts counting and will skip K-...read more
Ans. 
The Josephus problem involves eliminating individuals in a circle until only one remains, based on a specific counting rule.
• Start counting from position 1, skip K-1 individuals, eliminate the Kth person, and continue until only one person remains.

• The position of the last surviving person can be determined based on the initial numbering and the value of K.

• Example: For N=5 and K=2, the last person standing is at position
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Round duration - 75 minutes
Round difficulty - Medium

A google Doc was shared with us and we were supposed to write code there. 
Use of IDEs was not allowed so we had to write correct code on Google Docs which was later checked by them through online IDEs.
The Interviewer were friendly and observative and helped us through code if we made some silly error.

Q1. 
Largest Rectangle in Histogram Problem Statement
You are given an array/list HEIGHTS of length N, where each element represents the height of a histogram bar. The width of each bar is considered to be 1.
...read more
Ans. 
Compute the area of the largest rectangle that can be formed within the bounds of a given histogram.
• Iterate through the histogram bars and maintain a stack to keep track of increasing heights.

• Calculate the area of the rectangle formed by each bar as the smallest height in the stack times the width.

• Update the maximum area found so far.

• Time complexity can be optimized to O(N) using a stack-based approach.
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Q2. 
Boundary Traversal of a Binary Tree
Given a binary tree of integers, your task is to return the boundary nodes of the tree in Anti-Clockwise direction starting from the root node.
Input:
The first line ...read more
Ans. 
Return the boundary nodes of a binary tree in Anti-Clockwise direction starting from the root node.
• Traverse the left boundary nodes in top-down order

• Traverse the leaf nodes in left-right order

• Traverse the right boundary nodes in bottom-up order

• Handle duplicates in boundary nodes by including them only once
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Round duration - 60 minutes
Round difficulty - Medium

The face to face round was held on Google Meet where initially Interviewer asked a DS/Algo problem and then Later Manager Joined and asked about our resume projects in detail.
The time was 10:00 AM

Q1. 
Maximum Sum Rectangle Problem
Given an M x N matrix of integers ARR, your task is to identify the rectangle within the matrix that has the greatest sum of its elements.
Input:
The first line of input co...read more
Ans. 
Find the rectangle within a matrix with the greatest sum of elements.
• Iterate through all possible rectangles within the matrix and calculate their sums

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

• Keep track of the maximum sum found so far
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Q2. 
Rotated Array Minimum Finder
You are provided with a sorted array that has undergone 'K' rotations (the exact value of 'K' is unknown). A rotation involves shifting each element of the array to the right,...read more
Ans. 
Find the minimum number in a rotated sorted array efficiently.
• Use binary search to find the minimum element in the rotated array.

• Compare mid element with the last element to determine which half to search.

• Adjust the search space based on the comparison to find the minimum element efficiently.
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Q1. Find cousine of a node in a binary tree
Ans. 
Find the cousin of a node in a binary tree.
• Cousins are nodes at the same level but with different parents.

• Traverse the tree to find the level and parent of the given node.

• Traverse the tree again to find all nodes at the same level with different parents.

• Return the cousin node if found, else return null.
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Q2. One question on 2 pointer appraoch
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Round duration - 150 Minutes
Round difficulty - Easy

Debugging: This section had 7 debugging problems, which consist of code snippets that have some logical error that needs to be rectified.
Reasoning Ability: This section consists of some verbal reasoning questions and some aptitude questions.
Coding: This section consists of 2 coding problems.

Q1. 
Search in a Row-wise and Column-wise Sorted Matrix Problem Statement
You are given an N * N matrix of integers where each row and each column is sorted in increasing order. Your task is to find the positi...read more
Ans. 
Given a sorted N * N matrix, find the position of a target integer 'X'.
• Iterate over each row and column to search for the target integer 'X'.

• Utilize the sorted nature of the matrix to optimize the search process.

• Return the position of 'X' if found, else return '-1 -1'.
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Q2. 
Clone Linked List with Random Pointer Problem Statement
Given a linked list where each node has two pointers: one pointing to the next node and another which can point randomly to any node in the list or ...read more
Ans. 
Deep copy a linked list with random pointers and return its head. Validate if the cloned linked list is an exact copy of the original.
• Create a new node for each node in the original linked list and maintain a mapping between original and cloned nodes.

• Update the next and random pointers of the cloned nodes based on the mapping.

• Time complexity: O(N) where N is the number of nodes in the linked list.

• Space complexity: O(N)...read more
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Round duration - 60 minutes
Round difficulty - Medium

Around 30 candidates were shortlisted from my campus and over 150+ candidates were shortlisted from the university, and I was one of them. Then my first round of interviews was scheduled. I was pretty nervous before the interview.

Q1. 
Row Wave Form Problem Statement
Given a 2D array with dimensions N*M, your task is to read the array elements in a row-wise manner and return a linear array that stores the elements in a 'wave' pattern. S...read more
Ans. 
Given a 2D array, return a linear array storing elements in a 'wave' pattern.
• Read the array elements row-wise and store in wave pattern

• Alternate direction for storing elements from each row

• Handle edge cases like single row or single column arrays

• Use nested loops to iterate through the 2D array
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Q2. 
Construct a Strict Binary Tree Problem
Given an array/list of integers PRE representing the preorder traversal of a strict binary tree, and an array/list TYPENL with values 'L' or 'N', where 'L' denotes a...read more
Ans. 
Given preorder traversal and node type information, construct a strict binary tree and return the root node.
• Create a binary tree using preorder traversal and node type information

• Use recursion to construct the tree

• Handle leaf and non-leaf nodes separately

• Ensure each node has 0 or 2 children
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Round duration - 60 minutes
Round difficulty - Medium

My interviewer introduced himself in the beginning and asked for my introduction.

Q1. 
Alien Dictionary Problem Statement
You are provided with a sorted dictionary (by lexical order) in an alien language. Your task is to determine the character order of the alien language from this dictiona...read more
Ans. 
Given a sorted alien dictionary in an alien language, determine the character order of the language.
• Iterate through the dictionary to build a graph of character dependencies.

• Perform a topological sort on the graph to determine the character order.

• Return the character array representing the order of characters in the alien language.
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