Applied stacks/queues: undo, scheduling and breadth-first work

≈ 45 min

Applied stacks/queues: undo, scheduling and breadth-first work

Real algorithms use restricted structures to make order reliable. An undo feature stores prior actions on a stack. Breadth-first search uses a queue so it explores all nodes one edge away before nodes two edges away. A scheduler may use a priority queue where urgency and fairness are explicitly defined instead of silently relying on arrival order.

The structure does not solve the whole problem by itself. Breadth-first search also needs a visited set to avoid revisiting nodes and a rule for recording predecessors if a shortest path is required. Undo may use a second redo stack. Scheduling needs a policy for starvation so low-priority work is not postponed forever.

Worked reasoning. To find the fewest bus stops between two routes, enqueue the start stop, then repeatedly dequeue one stop and enqueue unvisited neighbours. The first time a stop is reached, it has been reached with the fewest edges because all earlier dequeues were no farther away. A stack would explore a deep route first and does not guarantee that property.

Exam lens. Link the claimed algorithmic property to the structure’s removal order. Do not say “use a queue because queues are efficient” without explaining the ordering need.

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Which statement is the most defensible principle for Applied stacks/queues: undo, scheduling and breadth-first work?

Use a stack, queue or priority structure because its removal order matches the algorithm’s required next action.

Treat an implementation detail as more important than the stated rule.

Assume one successful run proves the system is correct in every case.

Avoid recording assumptions, limits or failure conditions.

Enter the key term for Applied stacks/queues: undo, scheduling and breadth-first work. What traversal order explores all nearest neighbours before moving farther away?

Why is a queue preferable to a stack when finding the fewest links between two people in an unweighted network?

A queue explores all people at the current distance before farther people, so the first path found has the fewest links.

Use a larger class or broader permission without stating a rule.

Trust the first output and skip a boundary or failure check.

Hide the decision so affected people cannot inspect it.

Name the concise safeguard or principle that completes this lesson’s scenario: Why is a queue preferable to a stack when finding the fewest links between two people in an unweighted network?