LeetCode/cs/count-sub-islands.cs

public class Solution {
    private static readonly int WATER = 0;
    private static readonly int ISLAND = 1;

    private static readonly List<(int dy, int dx)> DIRECTIONS = [
        (1, 0),
        (-1, 0),
        (0, 1),
        (0, -1)
    ];

    private static void RunBFS(int[][] grid, int y0, int x0, int mark) {
        var q = new Queue<(int y, int x)>();

        // Handle the start
        grid[y0][x0] = mark;
        q.Enqueue((y0, x0));

        while (q.TryDequeue(out var next)) {
            var (y, x) = next;

            foreach (var (dy, dx) in DIRECTIONS) {
                var nextY = y + dy;
                var nextX = x + dx;

                if (nextY < 0 || nextY >= grid.Length || nextX < 0 || nextX >= grid[nextY].Length) {
                    continue;
                }

                if (grid[nextY][nextX] != ISLAND) {
                    continue;
                }

                grid[nextY][nextX] = mark;
                q.Enqueue((nextY, nextX));
            }
        }
    }

    private void MarkIslands(int[][] grid) {
        int mark = 1;

        for (int y = 0; y < grid.Length; ++y) {
            for (int x = 0; x < grid[y].Length; ++x) {
                if (grid[y][x] == ISLAND) {
                    RunBFS(grid, y, x, ++mark);
                }
            }
        }
    }

    private bool CheckSubIsland(int[][] grid1, int[][] grid2, int y0, int x0) {
        if (grid1[y0][x0] == WATER || grid2[y0][x0] == WATER) {
            return false;
        }

        var q = new Queue<(int y, int x)>();

        // Handle the start; sink the island to optimize for time complexity
        var expectedMark = grid1[y0][x0];
        grid2[y0][x0] = WATER;
        q.Enqueue((y0, x0));

        var isSubIsland = true;
        while (q.TryDequeue(out var next)) {
            var (y, x) = next;

            foreach (var (dy, dx) in DIRECTIONS) {
                var nextY = y + dy;
                var nextX = x + dx;

                // bounds
                if (nextY < 0 || nextY >= grid2.Length || nextX < 0 || nextX >= grid2[nextY].Length) {
                    continue;
                }

                // island continuation
                if (grid2[nextY][nextX] != ISLAND) {
                    continue;
                }

                // check if the mark matches
                if (grid1[nextY][nextX] != expectedMark) {
                    isSubIsland = false;
                }

                // sink to optimize
                grid2[nextY][nextX] = WATER;
                q.Enqueue((nextY, nextX));
            }
        }

        return isSubIsland;
    }

    public int CountSubIslands(int[][] grid1, int[][] grid2) {
        MarkIslands(grid1);

        int subislands = 0;
        for (int y = 0; y < grid2.Length; ++y) {
            for (int x = 0; x < grid2[y].Length; ++x) {
                if (CheckSubIsland(grid1, grid2, y, x)) {
                    ++subislands;
                }
            }
        }

        return subislands;
    }
}