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EasyPythonJavaScriptTypeScript

System Design — Hit Counter

System DesignQueueSliding Window

Build the counter behind a "requests in the last 5 minutes" metric.

solve(args, ops) builds new HitCounter() (no constructor arguments) and replays ops:

  • hit(timestamp) — record one hit at timestamp (seconds). Returns nothing.
  • count(timestamp) — return how many hits happened in the last 300 seconds, that is

hits with timestamp - 300 < hitTime <= timestamp.

Timestamps never go backwards. Several hits can share a timestamp. Keep memory bounded: drop
hits that have left the window instead of keeping the whole history (a queue works).

Example

hit(1) hit(2) hit(3) count(4)   → 3
hit(300) count(300)             → 4
count(301)                      → 3     // the hit at t=1 has expired

Sample tests

Test #1Classic example with an expiring hit
Input: [[],[["hit",1],["hit",2],["hit",3],["count",4],["hit",300],["count",300],["count",301]]]
Output: [null,null,null,3,null,4,3]
Test #2No hits yet
Input: [[],[["count",1]]]
Output: [0]
Test #3Hits sharing a timestamp expire together
Input: [[],[["hit",5],["hit",5],["hit",5],["count",5],["count",304],["count",305]]]
Output: [null,null,null,3,3,0]
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