Amazon Software Developer Intern Interview Questions, Process, and Tips

Round duration - 135 minutes
Round difficulty - Medium

Round 1- The online round had 4 sections:
1) Code Debugging Section: 7 questions to be debugged in 20 minutes (C/C++/Java)
2) Coding questions: There were 2 coding questions
3) Workstyle and Behavioural assessment: 20 minutes 
4) Reasoning Ability: 35 minutes (easy-medium)(Aptitude and Logical Reasoning Questions)

Q1. 
Character Formation Check
Determine if the second string STR2 can be constructed using characters from the first string STR1. Both strings may include any characters.
Input:
The first line contains an i...read more
Ans. 
Check if second string can be formed using characters from the first string.
• Iterate through each character in STR2 and check if it exists in STR1.

• Use a hashmap to store the frequency of characters in STR1 for efficient lookup.

• Return 'YES' if all characters in STR2 are found in STR1, otherwise return 'NO'.
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Q2. 
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 rows and columns 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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Round duration - 75 minutes
Round difficulty - Medium

It was a technical round. The interviewer mainly focused on Data Structures and Algorithms and asked me a few questions on operating systems.

Q1. 
Counting Distinct Elements in Every K-Sized Window
Given an array ARR of size N and an integer K, determine the number of distinct elements in every K-sized window of the array. A 'K' sized window is defi...read more
Ans. 
Implement a function to count distinct elements in every K-sized window of an array.
• Use a sliding window approach to keep track of distinct elements in each window

• Use a hashmap to store the frequency of elements in the current window

• Update the hashmap as you slide the window to the right and maintain the count of distinct elements

• Return the count of distinct elements for each window as an array
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Q2. 
Merge Two Sorted Linked Lists Problem Statement
You are provided with two sorted linked lists. Your task is to merge them into a single sorted linked list and return the head of the combined linked list.
...read more
Ans. 
Merge two sorted linked lists into a single sorted linked list with linear time complexity and constant space usage.
• Create a dummy node to start the merged list

• Compare the nodes of the two lists and link them accordingly

• Move the pointer to the next node in the merged list

• Handle cases where one list is empty or both lists are empty

• Return the head of the merged list
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Round duration - 145 minutes
Round difficulty - Easy

The online assessment consisted of four components, a code debugging section (20 minutes), a coding test (70 minutes), a workstyles assessment (20 minutes) and a reasoning ability section (35 minutes). Code debugging questions were pretty simple and straightforward. The coding test consisted of 2 questions, first was similar to two Sum problem. Second question was similar to find the critical edges in a graph. The workstyles assessment section contained certain behavioural questions.
Reasoning ability section consisted of aptitude questions. This round was basically to check the problem solving skills of the candidate.

Q1. 
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 difference between the target and the element exists in a hash set.

• If it exists, then print the pair (element, target-element), else add the element to the hash set.

• If no pair is found, print (-1, -1).
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Q2. 
Bridges in a Graph Problem Statement
In an undirected graph consisting of V vertices and E edges, your task is to identify all the bridges within the graph. A bridge is an edge which, when removed, result...read more
Ans. 
Identify bridges in an undirected graph by finding edges that, when removed, increase the number of connected components.
• Use depth-first search (DFS) to find bridges in the graph.

• Keep track of discovery time and low time for each vertex during DFS.

• An edge (u, v) is a bridge if low[v] > disc[u].

• Sort and print the bridge pairs in ascending order of vertices.

• Example: If the input graph has vertices 0, 1, 2, 3, 4 and ed...read more
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Round duration - 60 minutes
Round difficulty - Medium

I was asked two coding questions. First question was related to binary tree and second question was to implement LRU cache.
I had to code both questions. The interviewer asked me the time and space complexity for both the questions.

Q1. 
Circular Array Loop Problem Statement
Given a circular array ARR of size N consisting of positive and negative integers, determine if there is a cycle present in the array. You can make movements based on...read more
Ans. 
Determine if a circular array has a cycle present, following specific movement rules.
• Iterate through the array and check for cycles following the given movement rules.

• Keep track of visited indices to detect cycles.

• Handle both clockwise and counterclockwise movements separately.

• Return 'True' if a cycle is found, 'False' otherwise.
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Q2. 
LRU Cache Implementation
Design and implement a Least Recently Used (LRU) cache data structure, which supports the following operations:
• get(key) - Return the value of the key if it exists in the cach...read more
Ans. 
Implement a Least Recently Used (LRU) cache data structure with get and put operations.
• Use a combination of a hashmap and a doubly linked list to efficiently implement the LRU cache.

• Keep track of the least recently used item and update it accordingly when inserting new items.

• Ensure to handle the capacity constraint by evicting the least recently used item when the cache is full.

• For get operation, return the value of th
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Round duration - 60 minutes
Round difficulty - Medium

I was asked two coding questions and some conceptual questions about priority heap data structure. I was able to code the optimized approach for the first question. But second question was a little tricky for me as I had never heard it before. But I explained my approach confidently. The interviewer also helped me and finally I was able to code it.

Q1. 
Connect Ropes with Minimum Cost
Given 'N' ropes, each having different lengths, your task is to connect these ropes into one single rope. The cost to connect two particular ropes is equal to the sum of th...read more
Ans. 
Connect ropes with different lengths into one single rope with minimum cost by merging the smallest ropes first.
• Sort the lengths of ropes in ascending order.

• Merge the two smallest ropes at each step to minimize cost.

• Keep track of the total cost as you merge the ropes.

• Repeat the merging process until all ropes are connected.

• Return the total cost as the minimum cost to connect all ropes.
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Q2. 
Langford Pairing Problem Statement
Given a positive integer N, you are required to generate a list of integers of size 2N such that it contains all the integers from 1 to N, both included, twice. These in...read more
Ans. 
Generate Langford pairing for given N, return 'Valid' if correct, 'Invalid' if not.
• Generate a list of integers of size 2N with all integers from 1 to N twice.

• Arrange integers according to Langford pairing rules.

• Return 'Valid' if pairing is correct, 'Invalid' if not.

• If no valid pairing possible, return list with only -1 element.
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Round duration - 75 minutes
Round difficulty - Hard

This round was scheduled in the evening hours and all the participants were required to fill a form which was shared 15 minutes prior to the start of the online coding round. This form was filled out probably for the security reasons and to ensure that no one disinterested participant gives the test.

Q1. 
Longest Palindromic Substring Problem Statement
You are provided with a string STR of length N. The goal is to identify the longest palindromic substring within this string. In cases where multiple palind...read more
Ans. 
Given a string, find the longest palindromic substring, prioritizing the one with the smallest start index.
• Iterate through the string and expand around each character to find palindromes

• Keep track of the longest palindrome found so far

• Return the longest palindrome with the smallest start index
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Q2. 
Remove All The Palindromes Problem Statement
Given a string STR consisting only of lowercase letters, your task is to modify the string by replacing the minimum characters such that the string doesn’t con...read more
Ans. 
The task is to modify a string by replacing minimum characters to remove all palindromic substrings.
• Identify palindromic substrings in the given string

• Replace characters in the string to remove palindromic substrings

• Count the minimum number of characters replaced
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Round duration - 120 Minutes
Round difficulty - Medium

Round started at 6pm in the evening and had 4 sections.

Q1. 
Fire in the Cells Problem Statement
Given a matrix 'MAT' of size 'N' * 'M', where 'N' is the number of rows and 'M' is the number of columns. A cell with value '0' represents it is initially on fire (at t...read more
Ans. 
Given a matrix representing cells on fire or safe, determine if a person can reach an escape cell without stepping on fire.
• Check if the person can move to adjacent safe cells without stepping on fire

• If the person can reach an escape cell, return the time taken; otherwise, return -1

• Consider the constraints and note that escape cells are on matrix boundaries
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Q2. 
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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Round duration - 78 minutes
Round difficulty - Medium

The round started at 11 am and was conducted on amazon chime, They provided me a link to live sheet where I had to write all the code. Interviewer was very good at conducting a smooth interview and provided me small hints as I got stuck at some points.

Q1. 
Unique Element In Sorted Array
Nobita wants to impress Shizuka by correctly guessing her lucky number. Shizuka provides a sorted list where every number appears twice, except for her lucky number, which a...read more
Ans. 
Find the unique element in a sorted array where all other elements appear twice.
• Iterate through the array and XOR all elements to find the unique element.

• Use a hash set to keep track of elements and find the unique one.

• Sort the array and check adjacent elements to find the unique one.
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Q2. 
Level Order Traversal Problem Statement
Given a binary tree of integers, return the level order traversal of the binary tree.
Input:
The first line contains an integer 'T', representing the number of te...read more
Ans. 
Level order traversal of a binary tree is returned for each test case.
• Implement a function to perform level order traversal of a binary tree

• Use a queue data structure to keep track of nodes at each level

• Print the node values in level order traversal
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Round duration - 60 Minutes
Round difficulty - Medium

The round started at 4pm and was conducted on amazon chime, They provided me a link to live sheet where I had to write all the code.

Q1. 
Find Duplicate in Array Problem Statement
You are provided with an array of integers 'ARR' consisting of 'N' elements. Each integer is within the range [1, N-1], and the array contains exactly one duplica...read more
Ans. 
Identify the duplicate element in an array of integers.
• Iterate through the array and keep track of the frequency of each element using a hashmap.

• Return the element with a frequency greater than 1 as the duplicate.

• Time complexity can be optimized to O(N) using Floyd's Tortoise and Hare algorithm.

• Example: For input [3, 1, 3, 4, 2], the output should be 3.
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Round duration - 90 Minutes
Round difficulty - Medium

This round consisted of 20 MCQs and 2 coding problems. All the MCQs were from OOPs, DBMS & Algo, and aptitude. I solved all the questions in the given time. First coding question was of dynamic programming and second was of arrays. 20 students got shortlisted out of 134.

Q1. 
Trapping Rain Water Problem Statement
You are given a long type array/list ARR of size N, representing an elevation map. The value ARR[i] denotes the elevation of the ith bar. Your task is to determine th...read more
Ans. 
Calculate the total amount of rainwater that can be trapped between given elevations in an array.
• Iterate through the array and calculate the maximum height on the left and right of each bar.

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

• Sum up the trapped water at each bar to get the total trapped water for the entire array.
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Q2. 
Problem: Search In Rotated Sorted Array
Given a sorted array that has been rotated clockwise by an unknown amount, you need to answer Q queries. Each query is represented by an integer Q[i], and you must ...read more
Ans. 
Search for integers in a rotated sorted array efficiently.
• Use binary search to efficiently search for each query in the rotated sorted array.

• Keep track of the mid element and adjust the search based on the rotation.

• Return the index of the query if found, else return -1.
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Round duration - 60 Minutes
Round difficulty - Easy

The interviewer was really nice and supportive. After a short introduction, we went on to problem solving. He started with a easy question (as a warm up, I think). I coded and explained it in ~5 minutes. Then, he asked me another question, it would probably be medium level. I came up with a greedy solution using priority queue, explained, coded and tested it. We had some time left, so he asked me a follow up question. I was able to answer it, but I had no time left to code it. After that, I had a few minutes to ask any questions.

Q1. 
K Most Frequent Words Problem Statement
Given an array of N non-empty words and an integer K, return the K most frequent words sorted by their frequency from highest to lowest.
Example:
Input:
N = 6, K...read more
Ans. 
Return the K most frequent words from an array of words sorted by frequency.
• Use a hashmap to store word frequencies.

• Use a priority queue to keep track of the K most frequent words.

• Sort the words in the priority queue based on frequency and lexicographical order.

• Return the K most frequent words from the priority queue.
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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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