The fix itself is pretty mechanical once you see it, add a set, mark cells before enqueuing, done.
First explain the root cause: without tracking visited cells, BFS can revisit the same cell via different paths, leading to an infinite loop. Then describe the fix: add a visited set, mark cells when enqueued, and check before enqueueing. Finally, walk through how you would verify the fix with test cases and discuss the time/space complexity trade-offs.
Pro tip: Mention that marking visited at enqueue time (not dequeue) is crucial to prevent duplicate entries in the queue, which is a common subtle bug. Also, note that BFS guarantees shortest path in unweighted grids, so the visited set doesn't compromise correctness.
Explain that without a visited set, BFS can enqueue the same cell multiple times from different neighbors, causing cycles in the search and never terminating.
Add a set (or boolean matrix) to track visited cells. When exploring neighbors, check if a neighbor is already visited; if not, mark it visited and enqueue it.
Run the provided test cases, including edge cases like empty grid, no path, and large grids. Ensure the solver terminates and returns correct shortest paths.
Discuss time complexity O(V+E) and space O(V) for the visited set. Mention that using a set vs. modifying the grid in-place are trade-offs (memory vs. mutability).
Describe how BFS explores neighbors in all directions; without visited tracking, it can bounce between two adjacent cells indefinitely, creating an infinite loop.
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