hierarchical

k-means is the reflex answer, yet it silently assumes round, equal-size clusters and requires you to know k in advance. What earns credit is positioning each alternative by the specific assumption it drops, and knowing when to use it.

Forecasting a single series is textbook work; forecasting a million SKU-stores that have to reconcile, where stockouts cost more than overstock, is what the interview actually tests. The strong answer is global, hierarchical, and cost-aware.