I started with the recursive DFS and it went fine, but when they asked me to do the stack-based version I fumbled a bit on the order I was pushing neighbors.
Start by clarifying the problem and edge cases, then explain the core idea of traversing each unvisited '1' and exploring its connected component using DFS (recursive or iterative) or BFS. Compare recursive and iterative implementations, emphasizing how to mark visited cells to avoid cycles, and analyze time and space complexity. Finally, discuss handling large inputs and potential optimizations.
Pro tip: Mention that for very large grids, recursion may cause stack overflow, so an iterative approach with an explicit stack or BFS is safer; also note that modifying the grid in-place to mark visited cells saves space but may not be allowed if input must be preserved.
Confirm the definition of connected regions (4-directional), and discuss edge cases such as empty grid, grid with no 1s, or grid with all 1s. Also ask about input size constraints to choose the right approach.
Iterate through each cell; when a '1' is found and not visited, increment the region count and traverse all connected '1's using DFS or BFS, marking them as visited.
Describe a recursive function that marks the current cell as visited and recursively calls itself on all four adjacent cells that are '1' and not visited. Mention base cases and how to avoid revisiting.
Use an explicit stack (DFS) or queue (BFS) to explore the region. Push the starting cell, then while the stack/queue is not empty, pop a cell, mark it visited, and push all unvisited adjacent '1's.
Time complexity is O(m*n) since each cell is visited once. Space complexity is O(m*n) in the worst case for the recursion stack or queue. Discuss how to handle very large inputs, e.g., using iterative BFS to avoid stack overflow, or processing in chunks.
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