Software Engineer interview questions and what they really test
Engineering interviews are won on reasoning out loud: how you decompose a coding problem, scale a design, and debug under pressure. These questions cover the rounds most companies run and what interviewers listen for in each.
Technical Experience
What the interviewer is testing: the hardest technical problem they solved and how.
- Tell me about the hardest technical problem you personally solved. What made it hard?
- Describe a system you designed that later broke at scale. What did you miss and fix?
Problem Solving
What the interviewer is testing: reasoning through an algorithmic/data-structure problem out loud, complexity and edge cases.
- Design a data structure that returns the median of a growing stream in O(log n). Reason through it.
- Longest substring without repeating characters. Brute force, then optimize, then complexity.
- Dedupe 10 billion URLs that do not fit in memory. Walk the whole approach.
- Design an LRU cache with O(1) get and put. What breaks under concurrency?
- Given a huge log file, find the top-K most frequent IPs. Memory and time trade-offs.
- Serialize and deserialize a binary tree. Then make it work across languages.
System Design
What the interviewer is testing: designing a scalable system: data model, APIs, bottlenecks, trade-offs.
- Design a URL shortener at 100k req/s. Data model, APIs, hot keys, and the bottleneck you'd hit first.
- Design a live leaderboard for 50M users updating in real time. Consistency vs latency trade-offs.
- Design a distributed rate limiter. What happens during a network partition?
- Design the backend for a chat app like WhatsApp. Delivery guarantees and fan-out.
- Design a news feed for 500M users. Push vs pull, and how you keep it fresh.
- Design an image upload + CDN pipeline. Where do you cache, and how do you invalidate?
Debugging & Quality
What the interviewer is testing: how they debug production issues and think about reliability/testing.
- One endpoint's p99 jumped from 50ms to 2s overnight, deploys look clean. Find the cause.
- A service leaks memory in production but not in staging. Your investigation plan.
- Users report intermittent 500s that you cannot reproduce. How do you hunt it down?
- How do you decide what to unit-test vs integration-test in a service you own?
Behavioral
What the interviewer is testing: collaboration, disagreement with a decision, ownership of a failure.
- Tell me about a time you shipped something the data later proved was wrong. What did you do?
- Describe a decision where you disagreed with your director and had to defend your call.
- Tell me about the hardest prioritization trade-off you personally owned.
Rapid Fire
What the interviewer is testing: quick technical judgment calls.
- iOS or Android to launch first, and why in one line?
- Would you rather ship fast and iterate, or ship polished? Pick one.
- One metric to run all of YouTube by. Go.
- Kill a beloved feature or delay a launch by a month. Choose.
Can you answer these out loud, under follow-up pressure?
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