Coding problem: Sliding-Window Rate Limiter. An API lets each client make at most `limit` accepted requests in any window of `window_ms` milliseconds. You get the incoming `requests` as `[client_id, timestamp_ms]` pairs in time order. Decide, in order, whether each request is accepted, and return a list of booleans.

What the interviewer is looking for

• What a strong solution looks like: per client, a queue (deque) of accepted timestamps; for each request drop timestamps <= t - window_ms from the front, accept if fewer than limit remain, and only then push t. O(n) amortised time, O(clients × limit) space.
• Think aloud. Interviewers are judging how you reason, not only the final code.
• Be ready for: Memory is O(limit) per client — how do a sliding-window counter or a token bucket approximate this in O(1)? How do you enforce it across many API servers (Redis sorted sets or INCR with expiry, atomicity via Lua, clock skew)? What should a rejected response include so clients back off (429, Retry-After)?

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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