cs: add «1905. Count Sub Islands»

Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/count-sub-islands.cs

@@ -0,0 +1,110 @@
+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;
+    }
+}