Alibaba Group Big Data Engineer Interview Questions and Answers

Calculate the level sum of a binary tree.

• Traverse the tree level by level using BFS

• Add the values of nodes at each level

• Store the level sum in an array

• Return the array of level sums

Vertical sum of binary tree is the sum of all nodes in each vertical line of the tree.

• Traverse the tree in-order and keep track of the horizontal distance of each node from the root.

• Use a hash table to store the sum of nodes at each horizontal distance.

• Recursively traverse the left and right subtrees and update the hash table accordingly.

Calculate top 10 frequently used words on Facebook in 1 hour dynamically.

• Use Facebook API to fetch data

• Implement a script to count word frequency

• Sort the words based on frequency and return top 10

I am a software engineer with experience in developing web applications and a passion for problem-solving.

• Proficient in programming languages such as Java, Python, and JavaScript

• Experience with front-end frameworks like React and Angular

• Familiarity with back-end technologies such as Node.js and Spring Boot

• Strong understanding of data structures and algorithms

• Ability to work collaboratively in a team environment

There are 204 squares and 1296 rectangles on a standard 8x8 chess board.

• The number of squares can be calculated using the formula n(n+1)(2n+1)/6, where n is the number of rows/columns.

• The number of rectangles can be calculated using the formula n(n+1)m(m+1)/4, where n and m are the number of rows and columns respectively.

• For an 8x8 chess board, there are 64 squares of size 1x1, 49 squares of size 2x2, 36 squares of siz...read more

Implementing a shuffle function for a playlist of songs

• Create a new empty playlist

• Randomly select a song from the original playlist and add it to the new playlist

• Remove the selected song from the original playlist

• Repeat until all songs have been added to the new playlist

• Return the new shuffled playlist

Memory leak is a situation where a program fails to release memory it no longer needs.

• Memory leaks can cause a program to consume more and more memory over time, eventually leading to crashes or other issues.

• Memory leaks can be caused by programming errors such as not freeing memory after it is no longer needed.

• Tools like valgrind can be used to detect memory leaks in C and C++ programs.

• Examples of memory leaks include...read more

Arrays have fixed size and can lead to memory wastage and performance issues.

• Arrays have a fixed size and cannot be resized dynamically.

• Inserting or deleting elements in an array can be time-consuming.

• Arrays can lead to memory wastage if they are not fully utilized.

• Arrays can cause performance issues if they are too large and need to be traversed frequently.

• Arrays can also be prone to buffer overflow attacks.

• Example: A...read more

Function to hide text on mouse click in JavaScript

• Create a function that takes an element as input

• Add an event listener to the element for a mouse click

• Toggle the element's display property between 'none' and its original value

Find two elements in an unsorted array whose sum is equal to a given number x.

• Use a hash table to store the difference between x and each element in the array.

• Iterate through the array and check if the current element is in the hash table.

• Return the pair of elements that add up to x.

BST stands for Binary Search Tree, a data structure used for efficient searching and sorting operations.

• BST is a tree-like data structure where each node has at most two children.

• The left child of a node contains a value less than the parent node, while the right child contains a value greater than the parent node.

• BST allows for efficient searching and sorting operations with a time complexity of O(log n).

• Examples of a...read more

Number of possible BSTs with 2 and 3 nodes.

• For 2 nodes, only 2 BSTs are possible.

• For 3 nodes, 5 BSTs are possible.

• Number of BSTs can be calculated using Catalan numbers formula.

• Catalan(2) = 2, Catalan(3) = 5.

Answering the question about possible trees with two and three nodes.

• For two nodes, there is only one possible tree.

• For three nodes, there are three possible trees.

• The formula for calculating the number of possible trees with n nodes is (2n-3)!!.

• The double exclamation mark represents the double factorial function.

• The double factorial function is defined as n!! = n(n-2)(n-4)...(1 or 2).

B+ trees are balanced trees used for indexing and searching large amounts of data.

• B+ trees are similar to binary search trees but have multiple keys per node.

• They are commonly used in databases and file systems.

• B+ trees have a high fanout, reducing the number of disk accesses required for searching.

• They are also self-balancing, ensuring efficient performance even with large amounts of data.

• Example: In a database, a B+ ...read more

Yes, I have a few questions.

• Can you tell me more about the team I'll be working with?

• What is the company culture like?

• What are the biggest challenges the team is currently facing?

• Can you walk me through the development process for a typical project?

• What opportunities are there for professional growth and development?

I prefer hash tables for their constant time lookup and insertion.

• Hash tables are efficient for storing and retrieving data.

• They have constant time complexity for both insertion and lookup.

• They can be implemented using arrays or linked lists.

• Examples include Python's dictionary and Java's HashMap.

Yes, we can implement a stack using two queues.

• Push operation: Enqueue the element to the first queue.

• Pop operation: Dequeue all elements from the first queue and enqueue them to the second queue until the last element. Dequeue and return the last element. Swap the names of the queues.

• Top operation: Same as pop operation but don't dequeue the last element.

• Example: Push 1, 2, 3. Queue 1: 1 2 3. Queue 2: empty. Pop opera...read more

Find minimum no of platforms required to avoid collision between trains based on their arrival and departure times.

• Sort both arrays in ascending order

• Initialize platform count and max count to 1

• Loop through both arrays and compare arrival and departure times

• If arrival time is less than or equal to departure time, increment platform count

• Else, decrement platform count

• Update max count if platform count is greater than ma

JVM stands for Java Virtual Machine. It is an abstract machine that provides a runtime environment for Java programs.

• JVM is responsible for interpreting the compiled Java code and executing it.

• It provides platform independence by converting bytecode into machine-specific code.

• JVM has various components like class loader, bytecode verifier, and execution engine.

• Compiler converts source code into bytecode, while JVM exec...read more

I am a software engineer with experience in developing web applications and a passion for learning new technologies.

• Experienced in developing web applications using technologies such as Java, Spring, and Angular

• Passionate about learning new technologies and keeping up with industry trends

• Strong problem-solving skills and ability to work in a team environment

• Completed a Bachelor's degree in Computer Science from XYZ Uni

PayTM is an Indian e-commerce payment system and digital wallet company.

• PayTM was founded in 2010 by Vijay Shekhar Sharma.

• It started as a mobile recharge and bill payment platform.

• PayTM has expanded to offer services like online shopping, movie ticket booking, and travel bookings.

• It also offers a digital wallet that can be used to pay for various services and products.

• PayTM has over 350 million registered users and is ...read more

I would like to add a feature for splitting bills among friends.

• The feature would allow users to split bills for movies, dinners, etc.

• Users can select friends from their contact list and split the bill equally or unequally.

• The app would send a notification to the selected friends to pay their share.

• The feature would make it easier for users to split expenses and avoid awkward conversations.

• It would also encourage more

My favourite app is Spotify. I would like to see improvements in the algorithm for personalized playlists.

• Improved algorithm for personalized playlists

• Better integration with social media platforms

• Option to create collaborative playlists with friends

My favourite subject is Computer Science.

• I enjoy programming and problem-solving.

• I find algorithms and data structures fascinating.

• I am interested in artificial intelligence and machine learning.

• I like learning about new technologies and keeping up with industry trends.

I choose NIT Hamirpur because of its excellent academic reputation and beautiful campus.

• NIT Hamirpur has a strong focus on academics and research, which aligns with my career goals.

• The campus is located in a serene and picturesque location, which provides a conducive environment for learning.

• The faculty members are highly experienced and knowledgeable, and are always willing to help students.

• The college has state-of-th...read more

Count the number of continuous sequences with equal number of 0's and 1's in an array.

• Iterate through the array and keep track of the count of 0's and 1's encountered so far.

• Store the difference between the counts in a hash table and increment the count for that difference.

• If the difference is already present in the hash table, add the count to the existing value.

• Return the sum of all values in the hash table.

Count the number of palindromic substrings in a given string.

• A substring is a contiguous sequence of characters within a string.

• A palindrome is a string that reads the same backward as forward.

• Use dynamic programming to count all palindromic substrings.

• Time complexity can be reduced to O(n^2) using Manacher's algorithm.

Find the longest palindrome in a given string in linear time complexity.

• Use Manacher's algorithm to find the longest palindrome in a string in O(n) time complexity.

• The algorithm involves preprocessing the string to add special characters to handle even and odd length palindromes.

• Then, it uses a combination of dynamic programming and symmetry properties to find the longest palindrome.

• For example, in the string 'babad',

The value of pie is a mathematical constant approximately equal to 3.14159.

• Pie is the ratio of the circumference of a circle to its diameter.

• It is an irrational number, meaning it cannot be expressed as a finite decimal or fraction.

• Pie is commonly used in geometry and trigonometry calculations.

• The value of pie is often approximated as 3.14 or 22/7.

I am a passionate software engineer with experience in developing web applications using various technologies.

• Experienced in developing web applications using HTML, CSS, JavaScript, and frameworks like React and Angular

• Proficient in backend development with Node.js and databases like MongoDB and MySQL

• Familiar with version control systems like Git and deployment tools like Docker

• Strong problem-solving skills and ability

Yes, I believe I will be able to fit in the company based on my experience, skills, and values.

• I have experience working in diverse teams and adapting to different work cultures.

• I share the company's values and vision, which align with my own.

• I am eager to learn and contribute to the team, fostering a positive work environment.

• I have strong communication skills, which are essential for collaboration and teamwork.