DESIGN

The move

You get a blueprint: a class with a few methods and a promised cost for each. Snap a helper part onto the plain data (a second value, a map from key to node, a heap) so every call stays fast.

Spot it

The problem says design or implement a class, and each method must run in O(1) or O(log n), often on average.

Cost

Fast calls are paid for with extra memory, usually one entry per item: O(n) space.