Confluent Interview Process - Senior Senior Software Engineer II

Senior Software Engineer II at Confluent: an OA, two coding rounds and a system design round.

Round 1: Online assessment

Two problems, one easy and one hard. I don't remember the exact questions: one was basic string manipulation, the other a DP problem. Solved both.

Round 2: Coding

The interviewer said upfront they wanted clean, working code with the validations you'd expect in production: input validation, null checks and other relevant edge cases.

Problem 1: Given an N×N Sudoku board, determine whether it is valid

I used three 2D arrays of size N×10 to track rows, columns and sub-boxes. Follow-up: could I optimise the space? I suggested a single 2D array, at the cost of running separate loops for rows, columns and sub-boxes.

Problem 2: Using that validation, implement a Sudoku solver

I explained a recursion and backtracking approach, implemented it, and tested it with a few test cases covering corner scenarios.

Round 3: Coding

Problem 1: Implement the Unix tail -N command

Given a file, read and print the last N lines. I used a fixed-size queue of capacity N, keeping the most recent N lines while reading the file sequentially.

Problem 2: Optimise for a very large, continuously growing log file

I suggested reading the file backwards in fixed-size chunks, counting newline characters until there were enough to find where the last N lines start. Then seek to that offset and read forward.

I couldn't finish implementing this, I didn't have enough time left.

Round 4: System design

Design a temporary email service where email addresses and received emails are only valid for 10 minutes.

I completed the design and covered most of the deep dive areas. The interviewer was most interested in how I'd generate unique temporary email IDs. I went through several approaches with their pros and cons, including cryptographically secure random generation and word lists with numeric suffixes, using DB-level conditional writes to guarantee uniqueness.

Outcome

Rejected after this first system design round. The recruiter called to tell me they wouldn't be moving ahead.