The codebase was messy enough that I spent way too long just finding where BFS actually kicked off.
First, identify the bug by tracing the BFS algorithm: the start node must be marked visited immediately to prevent reprocessing. Then, fix the code by adding the missing visited marking, and finally, walk through a small graph test case that would fail without the fix (e.g., cycle or self-loop) to demonstrate correctness.
Pro tip: Emphasize that marking the start node as visited is crucial not only for correctness but also for efficiency, as it prevents infinite loops in cyclic graphs. Mention that this is a common off-by-one initialization error in BFS implementations.
Locate the initialization section of the BFS and note that the start node is not added to the visited set or marked as visited before enqueuing.
Describe how this bug can cause the start node to be revisited, leading to infinite loops in cyclic graphs or incorrect traversal order.
Add a line to mark the start node as visited (e.g., visited.add(start) or visited[start] = true) before adding it to the queue.
Select a simple graph that exposes the bug, such as a graph with a cycle or a self-loop, where the start node is reachable from itself.
Trace the BFS execution step-by-step, showing how the fixed code correctly visits each node once and terminates, while the buggy code would loop or produce wrong output.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.