An algorithm self-check tests progress, ordering, graph reachability and the invariants that justify a result.
Review the contracts
Review tree ordering, prefix membership, heap ordering before answering.
Choose and explain an answer
State the invariant before predicting the output. Separate a membership test from prefix reachability and a priority queue from sorted traversal.
Select an answer for every question, then check the result and read each explanation. Progress remains in this browser; this is a self-check, not a judged program submission or a credential.
Common Mistakes
A heap need not be globally sorted. A reachable trie prefix is not necessarily a stored complete key.