cs: add «1568. Minimum Number of Days to Disconnect Island»

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

cs/minimum-number-of-days-to-disconnect-island.cs

@@ -0,0 +1,87 @@
+public class Solution {
+    record Position(int x, int y) {
+        public bool InBounds(int width, int height)
+            => y >= 0 && y < height && x >= 0 && x < width;
+
+        public static IEnumerable<Position> Grid(int width, int height) {
+            for (var y = 0; y < height; ++y) {
+                for (var x = 0; x < width; ++x) {
+                    yield return new(x, y);
+                }
+            }
+        }
+
+        public static Position operator +(Position lhs, Position rhs)
+            => new(lhs.x + rhs.x, lhs.y + rhs.y);
+    }
+
+    private static readonly int WATER = 0;
+    private static readonly int ISLAND = 1;
+    private static readonly List<Position> DIRECTIONS = [
+        new Position(0, 1),
+        new Position(0, -1),
+        new Position(1, 0),
+        new Position(-1, 0),
+    ];
+
+    private void Explore(int[][] grid, bool[,] visited, Position cell) {
+        var (width, height) = (grid[0].Length, grid.Length);
+        bool toVisit(Position p) => p.InBounds(width, height) && grid[p.y][p.x] == ISLAND && !visited[p.y, p.x];
+
+        var q = new Queue<Position>();
+        q.Enqueue(cell);
+        visited[cell.y, cell.x] = true;
+
+        while (q.TryDequeue(out var p)) {
+            foreach (var direction in DIRECTIONS) {
+                var next = p + direction;
+                if (toVisit(next)) {
+                    q.Enqueue(next);
+                    visited[next.y, next.x] = true;
+                }
+            }
+        }
+    }
+
+    private int CountIslands(int[][] grid) {
+        var (width, height) = (grid[0].Length, grid.Length);
+
+        var visited = new bool[height, width];
+
+        var islands = 0;
+        foreach (var cell in Position.Grid(width, height)) {
+            if (!visited[cell.y, cell.x] && grid[cell.y][cell.x] == ISLAND) {
+                Explore(grid, visited, cell);
+                ++islands;
+            }
+        }
+
+        return islands;
+    }
+
+    public int MinDays(int[][] grid) {
+        var (width, height) = (grid[0].Length, grid.Length);
+        var islands = CountIslands(grid);
+
+        if (islands != 1) {
+            return 0;
+        }
+
+        foreach (var p in Position.Grid(width, height)) {
+            if (grid[p.y][p.x] == WATER) {
+                continue;
+            }
+
+            grid[p.y][p.x] = WATER;
+            islands = CountIslands(grid);
+
+            if (islands != 1) {
+                return 1;
+            }
+
+            grid[p.y][p.x] = ISLAND;
+        }
+
+        return 2;
+    }
+}