Count Sub Islands

A sub-island is a complete land component of grid2 whose every cell is also land in grid1. Iterate grid2; when an unvisited land cell is found, DFS/BFS the whole component, marking grid2 visited or mutating it to water. Carry a boolean valid initialized true and set it false if any visited coordinate has grid1[r][c] equal to zero. Do not stop the traversal when invalid, because all of that component still must be marked to avoid recounting it. Add one to the answer only after the component completes and valid is still true. Clarify four-directional adjacency and whether mutation is allowed. Each grid cell is visited at most once, so time is O(RC) and extra space is O(RC) worst case for visited/queue or recursion.