Amazon Software Developer Intern Interview Questions, Process, and Tips

Round duration - 140 minutes
Round difficulty - Medium

Timing: 7 pm

Q1. 
Minimum Cost to Cross a Grid
You are given a 2D grid consisting of 'N' rows and 'M' columns. Each cell in the grid has a certain cost associated with passing through it. Your goal is to find the minimum c...read more
Ans. 
Find the minimum cost to travel from top-left to bottom-right corner of a grid.
• Use dynamic programming to keep track of minimum cost to reach each cell.

• Start from top-left corner and iterate through the grid to calculate minimum cost.

• Consider all possible movements (UP, DOWN, LEFT, RIGHT) while staying within grid boundaries.

• Update the cost for each cell by considering the minimum cost of reaching adjacent cells.

• The fi...read more
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Q2. 
Sub Matrices With Sum Zero Problem Statement
You are provided with a square matrix 'MAT' of order N. Your task is to determine the number of non-empty sub-matrices where the sum of all elements within the...read more
Ans. 
Count the number of sub-matrices with sum zero in a square matrix.
• Iterate over all possible sub-matrices and calculate the sum of elements within each sub-matrix.

• Use a hashmap to store the prefix sum of rows or columns to efficiently calculate the sum of sub-matrices.

• Check for sub-matrices with sum zero and increment the count accordingly.

• Return the total count of sub-matrices with sum zero.
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Round duration - 90 minutes
Round difficulty - Easy

Timing : 2 pm
The interviewer was very friendly.

Q1. 
Boundary Values of a Binary Tree Problem Statement
Given a binary tree of integers, your task is to print the boundary nodes of this binary tree in an anti-clockwise direction starting from the root node.
...read more
Ans. 
Print the boundary nodes of a binary tree in an anti-clockwise direction starting from the root node.
• Traverse the left boundary nodes in a top-down manner

• Traverse the leaf nodes from left to right

• Traverse the right boundary nodes in a bottom-up manner

• Avoid duplicates while printing the boundary nodes
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Q2. 
Segregate Even and Odd Nodes in a Linked List
You are given the head node of a singly linked list head. Your task is to modify the linked list so that all the even-valued nodes appear before all the odd-v...read more
Ans. 
Reorder a singly linked list such that even-valued nodes appear before odd-valued nodes while preserving the original order.
• Create two separate linked lists for even and odd nodes.

• Traverse the original list and append nodes to their respective lists based on their values.

• Finally, merge the even list followed by the odd list to get the desired order.
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Round duration - 180 Minutes
Round difficulty - Medium

Round 1 was a coding test which consisted of 4 sections.
1st section.
There were 7 debugging questions which was to be solved in 20 minutes.
A code was given which we have to debug and it should pass all the test cases. If you have a basic understanding of loops and concepts then this section is easy.

2nd section.
There were 2 coding questions which were to be done in 70 minutes.

3rd section
This section was more about behaviour and nature.

4th section 
There were 35 reasoning based questions, which was to be done in 24 minutes.

Q1. 
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 without using additional space.
• Create a dummy node to start the merged list

• Compare the nodes of both 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

• Time complexity: O(n), Space complexity: O(1)
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Q2. 
Word Break Problem Statement
You are given a list of N strings called A. Your task is to determine whether you can form a given target string by combining one or more strings from A.
The strings from A c...read more
Ans. 
Given a list of strings, determine if a target string can be formed by combining one or more strings from the list.
• Iterate through all possible combinations of strings from the list to form the target string.

• Use recursion to try different combinations of strings.

• Check if the current combination forms the target string.

• Return true if a valid combination is found, otherwise return false.
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Round duration - 60 minutes
Round difficulty - Medium

Round 2 was 1-hour long technical interview. It was on 20th June 2020, from 10 AM to 11 AM. The round started with the introductions and then the discussion on projects.

Q1. 
Add Two Numbers as Linked Lists
You are given two singly linked lists, where each list represents a positive number without any leading zeros.
Your task is to add these two numbers and return the sum as ...read more
Ans. 
Add two numbers represented as linked lists and return the sum as a linked list.
• Traverse both linked lists simultaneously while adding corresponding nodes and carry over the sum if needed

• Handle cases where one linked list is longer than the other by adding remaining nodes with carry

• Create a new linked list to store the sum and return its head node
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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. 
Determine if it is feasible to complete all courses with given prerequisites.
• Create a graph representation of the courses and prerequisites.

• Check for cycles in the graph using topological sorting.

• If there are no cycles, all courses can be completed.

• Example: For N=2 and prerequisites=[[1, 2]], output is 'Yes'.
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Q3. What are the ACID properties in a database management system?
Ans. 
ACID properties are a set of properties that guarantee the reliability of transactions in a database management system.
• Atomicity: Ensures that either all operations in a transaction are completed successfully or none at all.

• Consistency: Ensures that the database remains in a consistent state before and after the transaction.

• Isolation: Ensures that the execution of multiple transactions concurrently does not interfere w...read more
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Round duration - 120 minutes
Round difficulty - Medium

The round was based on DSA and CS fundamentals.
Three DSA questions were asked, in each of them I had to properly explain my approach and then code the entire solution.
After the DSA questions, I was asked questions on CS fundamentals.

Q1. 
Triplet Count in a Sorted Doubly Linked List Problem
Count the number of triplets in a sorted doubly linked list such that their sum equals a given integer 'X'. The list is composed of distinct node value...read more
Ans. 
Count the number of triplets in a sorted doubly linked list such that their sum equals a given integer 'X'.
• Iterate through the linked 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 while traversing the list.

• Return the final count of triplets that sum up to 'X'.
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Q2. 
Number of Islands Problem Statement
Given a non-empty grid of 0s and 1s, determine the number of distinct islands. An island is a collection of '1's (land) connected horizontally, vertically, or diagonall...read more
Ans. 
The task is to determine the number of distinct islands in a grid of 0s and 1s connected horizontally, vertically, or diagonally.
• Iterate through the grid and perform depth-first search (DFS) to explore each island

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

• Increment the count of islands whenever a new island is encountered
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Q3. 
Flip Bits Problem Explanation
Given an array of integers ARR of size N, consisting of 0s and 1s, you need to select a sub-array and flip its bits. Your task is to return the maximum count of 1s that can b...read more
Ans. 
Given an array of 0s and 1s, find the maximum count of 1s by flipping a sub-array at most once.
• Iterate through the array and keep track of the maximum count of 1s obtained by flipping a sub-array.

• Consider flipping a sub-array only when it results in increasing the count of 1s.

• Update the maximum count of 1s as you iterate through the array.

• Return the maximum count of 1s obtained after considering all possible sub-arrays
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Round duration - 55 minutes
Round difficulty - Medium

It was a coding interview that started with introducing myself. Two questions from data structures were asked.

Q1. 
Matrix Median Problem Statement
You are provided with a matrix containing 'N' rows and 'M' columns filled with integers. Each row is sorted in non-decreasing order. Your task is to find the overall median...read more
Ans. 
Find the overall median of a matrix with sorted rows.
• Merge all elements of the matrix into a single sorted array

• Calculate the median of the merged array

• Handle odd number of elements by directly finding the middle element
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Q2. 
Count Triplets in a Sorted Doubly Linked List
You are provided with an integer X and a non-decreasing sorted doubly linked list consisting of distinct nodes.
Your task is to find and return the count of ...read more
Ans. 
Count triplets in a sorted doubly linked list that sum up to a given value.
• Iterate through the list and for each node, find pairs that sum up to X-nodeValue.

• Use two pointers approach to find pairs efficiently.

• Keep track of count of triplets found while iterating through the list.
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Round duration - 180 minutes
Round difficulty - Medium

Were were given the test link and 3 days, we could take the test anytime during these three days and diffrent candidates got different questions in their tests. 
The online assessment consists of the following sections:
Code Debug: In this portion of the online assessment, you will be asked to find bugs in seven pieces of code. 

Coding Test: This portion of the online assessment consists of DS algo questions. There were 2 questions.

Work Styles Assessment: This is built around Amazon’s Leadership Principles, you have to choose what extent a provided statement represents your work style

Logical Ability: During this logical ability section, you will be asked a series of problem solving multiple choice questions.

Once you move to next section, you cannot go back to previous one.

Q1. 
Pair Sum Problem Statement
You are given an integer array 'ARR' of size 'N' and an integer 'S'. Your task is to find and return a list of all pairs of elements where each sum of a pair equals 'S'.
Note:
...read more
Ans. 
Find pairs of elements in an array that sum up to a given value, sorted in a specific order.
• Iterate through the array and for each element, check if the complement (S - current element) exists in a hash set.

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

• Sort the result list based on the criteria mentioned in the problem statement.
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Q2. 
Running Median Problem
Given a stream of integers, calculate and print the median after each new integer is added to the stream.
Output only the integer part of the median.
Example:
Input:
N = 5  
Stre...read more
Ans. 
Calculate and print the median after each new integer is added to the stream.
• Use a min heap to store the larger half of the numbers and a max heap to store the smaller half.

• Keep the sizes of the two heaps balanced to efficiently calculate the median.

• If the total number of elements is odd, the median is the top element of the max heap.

• If the total number of elements is even, the median is the average of the top elements
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Round duration - 90 minutes
Round difficulty - Easy

I solved almost all the MCQs correctly and passed all the test cases for both the questions.

Q1. 
Max GCD Pair Problem Statement
Given an array of positive integers, determine the Greatest Common Divisor (GCD) of a pair of elements such that it is the maximum among all possible pairs in the array. The...read more
Ans. 
Find the maximum GCD of a pair of elements in an array of positive integers.
• Iterate through all pairs of elements in the array to find their GCD.

• Keep track of the maximum GCD found so far.

• Use Euclidean algorithm to calculate GCD efficiently.

• Return the maximum GCD value found.
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Q2. 
Kevin and his Fruits Problem Statement
Kevin has 'N' buckets, each consisting of a certain number of fruits. Kevin wants to eat at least 'M' fruits. He is looking to set a marker as high as possible such ...read more
Ans. 
Find the marker value needed for Kevin to eat at least 'M' fruits from 'N' buckets.
• Iterate through each bucket and calculate the difference between the number of fruits and the marker value needed to reach 'M'.

• Update the marker value to be the maximum of the current marker value and the difference calculated in the previous step.

• Return the final marker value as the output.
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Round duration - 60 minutes
Round difficulty - Medium

Q1. 
Bracket Number Problem Statement
Given a string S that contains some brackets, your task is to print the number associated with each bracket pair.
Example:
Input:
S = (pq)() 
Output:
1 1 2 2
Explana...read more
Ans. 
Given a string with brackets, print the number associated with each bracket pair.
• Iterate through the string and maintain a stack to keep track of opening brackets

• Assign a number to each pair of opening and closing brackets

• Print the assigned numbers for each bracket pair
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Q2. 
Least Common Ancestor of Two Nodes
You are given an arbitrary binary tree with 'N' nodes, where the nodes are numbered with values in the range of integers. Given two specific nodes, 'x' and 'y', your tas...read more
Ans. 
Find the least common ancestor of two nodes in a binary tree.
• Traverse the tree to find the paths from the root to each node, then compare the paths to find the LCA.

• Use recursion to traverse the tree efficiently and find the LCA.

• Handle cases where one or both nodes are not present in the tree by checking for NULL references.

• The LCA is the node where the paths to the two nodes diverge in the tree.
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Round duration - 45 minutes
Round difficulty - Medium

Q1. 
Running Median Problem
Given a stream of integers, calculate and print the median after each new integer is added to the stream.
Output only the integer part of the median.
Example:
Input:
N = 5  
Stre...read more
Ans. 
Calculate and print the median after each new integer is added to a stream of integers.
• Use a min heap to store the larger half of the numbers and a max heap to store the smaller half.

• Keep the sizes of the two heaps balanced to efficiently calculate the median.

• If the total number of elements is odd, the median is the top element of the max heap.

• If the total number of elements is even, the median is the average of the to
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Q2. 
Detect Cycle in a Directed Graph
Determine if a given directed graph contains a cycle. Your function should return true if the graph contains at least one cycle, and false otherwise.
Input:
The first li...read more
Ans. 
Detect cycle in a directed graph and return true if cycle exists, false otherwise.
• Use Depth First Search (DFS) to detect cycles in the directed graph.

• Maintain a visited array to keep track of visited vertices and a recursion stack to keep track of vertices in the current path.

• If a vertex is visited and is in the recursion stack, then a cycle exists.

• Return true if a cycle is found, false otherwise.
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