BFS or DFS from the start, only expanding to neighbors with strictly lower height.
Clarify the problem constraints and edge cases, then propose a reverse BFS/DFS from the starting cell to find all reachable cells. Discuss time and space complexity, and compare with alternative approaches like forward DFS with memoization.
Pro tip: Mention that reversing the flow direction simplifies the traversal because you can start from the given cell and explore neighbors with higher or equal elevation, avoiding the need to track visited cells in multiple directions. This also naturally handles cycles and ensures each cell is visited once.
Ask about grid size, elevation range, whether diagonal moves are allowed, and if the starting cell is included in the result. Confirm that water flows only to strictly lower neighbors.
Propose a reverse BFS/DFS starting from the given cell, exploring neighbors with elevation >= current cell's elevation. Alternatively, consider forward DFS with memoization, but note the reverse approach is simpler and more efficient.
Initialize a queue with the starting cell, mark it visited, and while the queue is not empty, pop a cell and check its 4 neighbors. If a neighbor is unvisited and its elevation is >= the current cell's elevation, add it to the queue and mark visited.
State that time complexity is O(m*n) where m and n are grid dimensions, as each cell is visited at most once. Space complexity is O(m*n) for the visited set and queue in the worst case.
Mention handling of empty grid, single cell, all equal elevations, and disconnected regions. Compare BFS vs DFS (both work, BFS avoids recursion depth issues) and note that the reverse approach avoids redundant checks.
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