System design interview question
"Design a trending system — the top 10 most-viewed videos, or the top 100 hashtags right now. A million events per second, hundreds of millions of distinct items, and the answer must be current within a minute. Tell me how you find the heavy hitters without counting everything."
streaming algorithms for heavy hitters, which is one of the few system-design questions with a genuinely algorithmic core. The distinguishing content is: why exact counting is infeasible, what a Count-Min Sketch buys and costs, and — the part most candidates miss — that "trending" means rate of change, not total count.
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