Coding problem: Diff Lists by ID. A UI keeps a list of items, each an object with a unique integer `id` plus other fields. Given the list `before` and the list `after`, return an object `{"added": [...], "removed": [...], "changed": [...]}` of ids:

What the interviewer is looking for

• What a strong solution looks like: index `before` by id in a hash map; walk `after` collecting added and changed (deep equality on the item), then walk `before` for removed using a set of the ids in `after`. O(n + m) plus the cost of comparing items.
• Think aloud. Interviewers are judging how you reason, not only the final code.
• Be ready for: Also report moves with the fewest operations (longest increasing subsequence — what React's keyed reconciliation approximates). Return which fields changed per id (a patch). What if ids can be missing or duplicated? How would you diff two 10-million-row tables (hash per row, sort-merge)?

A structure to follow: Clarify, plan, code, test, analyse

1. Step 1

   Clarify

   Restate the problem, ask about empty input, duplicates and size limits, and agree on one example.

2. Step 2

   Plan

   Say the simple approach first and its cost, then the better one. Explain why before you type.

3. Step 3

   Code

   Write small named steps and talk through each. A clear solution beats a clever one.

4. Step 4

   Test

   Walk your own example through the code, then an edge case. Fix what you find calmly.

5. Step 5

   Analyse

   State time and space cost in plain words and say what you would change for much larger input.

This is guidance on shape, not a script. Use your own real experience and facts; never invent details to fill a gap. See the STAR guide and answer frameworks.

Common slips

• Starting to code before agreeing what the answer must be.
• Going silent when stuck instead of saying what you are trying.
• Never running your own example through the code.

How to practise it

Solve it once in the editor without hints, then explain your solution aloud in under two minutes.

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