The fix itself isn't hard once you see it.
First, clarify the expected behavior of BFS/DFS on a grid and the symptoms of the bug (infinite loop or repeated processing). Then, systematically trace the algorithm to identify common pitfalls like missing visited checks or incorrect queue/stack management, and propose a fix with a test case.
Pro tip: Demonstrate a methodical debugging process by walking through a small example grid, showing exactly where the algorithm fails, and explaining how your fix prevents revisiting cells. This shows you can not only fix code but also validate it.
Restate the goal: BFS/DFS should explore each reachable cell at most once and terminate, returning a path if found. Identify the symptoms: infinite loop or repeated processing indicates cells are being revisited.
Check for missing or incorrect visited set, improper marking of visited cells (e.g., marking after dequeue instead of before enqueue), and incorrect neighbor iteration that might allow revisiting.
Simulate the algorithm on a tiny grid (e.g., 2x2) to observe where a cell gets processed multiple times. This pinpoints the exact line causing the issue.
Add or correct the visited check: mark cells as visited when they are added to the queue/stack, not when they are processed. Ensure the visited set is checked before adding neighbors.
Test with the small example and consider edge cases like unreachable targets, empty grids, or obstacles. Confirm termination and correct path return.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.