Microsoft L61 || || 11 July D - Microsoft

Summary

I successfully navigated multiple interview rounds for a Microsoft L61 position, which included Data Structures & Algorithms, System Design, and a Hiring Manager discussion, ultimately leading to an offer.

Full Experience

My interview journey for the Microsoft L61 position started with an Online Assessment, followed by several on-site rounds that took place after a three-week interval.

Round 1: Online Assessment

This round had two questions, which I recall being of medium-hard difficulty, and I had 90 minutes to solve them. Unfortunately, I don't remember the exact problems at this time.

Round 2: Data Structures & Algorithms

This was a focused DSA round.

Q1: Candy Crush Stack-based Question on Strings
The problem involved processing a string like s = "abccbd", where I had to remove consecutive duplicate characters, with a maximum occurrence of two. For instance, in "abccbd", the two 'c's would be removed first, leaving "abbd", and then the two 'b's would be removed, resulting in "ad". I was asked to code the solution for this.
The follow-up question explored how to handle multiple characters with similar removal rules, and they primarily wanted an approach and a dry run.
Q2: Multithreaded Counter
I was tasked with implementing get and put functions within a proper class structure for a multithreaded counter. This involved ensuring thread safety.

Round 3: System Design

This round primarily focused on a traffic logging system, which was based on a project from my resume. It's crucial to be hands-on with all resume projects, as they often form the basis of design questions.

I discussed functional and non-functional requirements, presented high-level architecture diagrams, explained how unique log messages could be handled through Kafka (and why Kafka was suitable), addressed idempotency, and talked about database selection, specifically comparing time-series databases with other options.

Round 4: Data Structures & Algorithms

Another DSA round with a challenging problem.

Q: Prefix Lock/Unlock System
I needed to implement lock and unlock functions for strings with a specific rule: a string or any of its prefixes cannot be locked if a prefix is already locked. Once unlocked, it can be locked again.
For example:
- lock("abc") returns 1 (locked)
- lock("def") returns 1 (locked)
- lock("abz") returns 0 (because "abc" is a locked prefix)
- unlock("abc") returns 1 (unlocked)
- lock("abz") now returns 1 (can be locked)
The follow-up was to make this system multithreaded.

Round 5: Hiring Manager Round

This round lasted about 30 minutes. I was asked several behavioral questions, discussed my previous projects in detail, and had a conceptual discussion on system design topics like horizontal versus vertical scaling.

After a three-week wait, I received the offer and was selected!

Interview Questions (5)

Candy Crush String Removal

Data Structures & AlgorithmsMedium

Given a string s, implement a function to remove consecutive duplicate characters. A character can occur a maximum of twice. For example, if s = "abccbd", first all 'c's will be popped up continuously, leaving 'abbd'. Then, the 'bb' will be popped up, leaving 'ad'.

Multithreaded Counter

Data Structures & AlgorithmsMedium

Implement a multithreaded counter with get and put functions. The implementation should ensure thread safety and proper class structure for concurrent access.

Traffic Logging System Design

System DesignHard

Design a traffic logging system. This involved discussing functional and non-functional requirements, high-level architecture diagrams, strategies for handling unique log messages (e.g., using Kafka), addressing idempotency, and making appropriate database selections (e.g., time-series vs. other database types). The discussion was based on a project I had listed on my resume.

Prefix Lock/Unlock System

Data Structures & AlgorithmsHard

Implement lock(string s) and unlock(string s) functions. The rules are:

lock(s) returns 1 if s is successfully locked. It returns 0 if s or any of its prefixes are already locked.
unlock(s) returns 1 if s was successfully unlocked. It returns 0 if s was not locked.

Example:

lock("abc") -> 1
lock("def") -> 1
lock("abz") -> 0 (because 'abc' is locked as a prefix)
unlock("abc") -> 1
lock("abz") -> 1 (now can be locked)

The problem also included a follow-up to make the solution multithreaded.

Behavioral and System Design Concepts Discussion

Behavioral

The Hiring Manager round involved various behavioral questions, in-depth discussions about previous projects mentioned on my resume, and conceptual questions on system design, such as comparing Horizontal vs. Vertical scaling.

Preparation Tips

I focused my preparation on Data Structures & Algorithms, paying particular attention to stack-based problems, multithreading and concurrency concepts (like mutexes, shared mutexes, and condition variables), and Trie data structures for efficient prefix operations. For System Design, I reviewed core architectural patterns, practiced designing systems from scratch, and made sure I could articulate functional and non-functional requirements, justify technology choices like Kafka, and discuss database selection strategies, including time-series databases.

Microsoft L61 || Interview Experience || 11 July drive