Polymorphism, vtables, and generic containers in C++
Binary heaps and priority queues
Array-backed complete binary trees, min-heap properties, sift-up and sift-down mechanics, linear-time heapify, and priority queue ordering.
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This one opens once you have finished RAII, objects, and lifecycles in C++.
Progress is kept against your account, so finishing a module needs one. Everything up to here is open without.