Coding problem: Merge Booked Slots. A calendar holds booked slots as `[start, end]` pairs of integers (minutes from midnight), in no particular order. Merge every group of overlapping or touching slots and return the merged slots sorted by start.

What the interviewer is looking for

• What a strong solution looks like: sort by start, then sweep, extending the last merged slot while start <= last.end (end = max(last.end, end)). O(n log n) time, O(n) for the output.
• Think aloud. Interviewers are judging how you reason, not only the final code.
• Be ready for: Given the merged busy slots, list the free slots between 09:00 and 17:00. How many rooms are needed so overlapping meetings never share one (sweep line / min-heap of end times)? Insert one new slot into an already merged list in O(log n + k).

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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