Software engineer interview questions

What they're checking

Whether you can get from a brute-force idea to an efficient one, and explain why it is efficient.

A strong answer covers

1. Restate the problem and ask about edge cases, such as an empty string or which characters can appear.
2. Mention the brute-force check of every pair, then improve on it by counting each character in a hash map.
3. Scan the string a second time and return the first character with a count of one.
4. State the cost (O(n) time and O(k) extra space for k distinct characters), then test it on a short example.

What they're checking

How you turn a vague prompt into requirements, then make and justify trade-offs at scale.

A strong answer covers

1. Pin down requirements first, such as the ratio of reads to writes, expected volume, custom aliases and expiry.
2. Sketch the core pieces (an API, a way to generate short keys and a key-value store) and say why you chose each.
3. Handle the read-heavy load with a cache in front of the store, and explain what happens on a cache miss.
4. Close with the weak points you would watch, such as hot keys or running out of short keys, and how you would monitor them.

What they're checking

Your debugging method rather than the bug itself, and whether you fixed the cause as well as the symptom.

A strong answer covers

1. Set the scene in a sentence or two: the system, the symptom and why it mattered.
2. Walk through how you narrowed it down, such as reproducing it, reading logs and bisecting recent changes.
3. Name the root cause plainly, including any part of it that was your own mistake.
4. End with what you changed so it could not happen again, such as a test, an alert or a design fix.

What they're checking

The scope of your own contribution, and whether you can explain technical work to a mixed audience.

A strong answer covers

1. Describe the project and the problem it solved in two sentences, without jargon.
2. Be specific about what you owned, as opposed to what the team did.
3. Pick one hard decision or obstacle and explain how you handled it.
4. Give the result in numbers if you have them, such as latency, users or hours saved.

What they're checking

Whether you can push back with evidence, stay constructive and commit once a decision is made.

A strong answer covers

1. Describe the decision and why you disagreed, in terms of risk or cost rather than taste.
2. Explain how you raised it, such as a written proposal, a prototype or data from a test.
3. Say what was decided, even if it went against you.
4. Share what happened next and what you learned about making your case.