Coding problem: Totals by Category. Each record is an object `{"category": string, "amount": integer or null}` describing one sale, with the amount in cents. Return an object that maps each category to the sum of its amounts.

What the interviewer is looking for

• What a strong solution looks like: one pass with a hash map — when amount isn't null, total[c] = total.get(c, 0) + amount. O(n) time, O(k) space for k categories.
• Think aloud. Interviewers are judging how you reason, not only the final code.
• Be ready for: Return the top 3 categories by total. How would you compute this over billions of records on many machines (partial aggregates, map-reduce, combiners)? Write it in SQL — how does SUM treat NULL there?

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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