cs: add «827. Making A Large Island»

URL:	https://leetcode.com/problems/making-a-large-island/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/making-a-large-island.cs

@@ -0,0 +1,63 @@
+using Coord = (int x, int y);
+
+public class Solution {
+    private static readonly List<Coord> DIRS = [(0, 1), (0, -1), (1, 0), (-1, 0)];
+
+    private static bool InBounds(int[][] grid, Coord pos)
+        => pos.y >= 0 && pos.y < grid.Length && pos.x >= 0 && pos.x < grid[pos.y].Length;
+
+    private static Coord Add(Coord pos, Coord d) => (pos.x + d.x, pos.y + d.y);
+
+    private static IEnumerable<Coord> Coords(int[][] grid) =>
+        Enumerable
+        .Range(0, grid.Length)
+        .SelectMany(
+            y => Enumerable.Range(0, grid[y].Length).Select(x => (x, y))
+        );
+
+    private static int ExploreIsland(int[][] grid, int island, Coord pos) {
+        if (!InBounds(grid, pos) || grid[pos.y][pos.x] != 1) {
+            return 0;
+        }
+
+        grid[pos.y][pos.x] = island;
+        return 1 + DIRS.Sum(
+            d => ExploreIsland(grid, island, Add(pos, d))
+        );
+    }
+
+    public int LargestIsland(int[][] grid) {
+        var islands = new Dictionary<int, int>();
+        var islandId = 2;
+
+        // Discover islands
+        foreach (var pos in Coords(grid).Where(pos => grid[pos.y][pos.x] == 1)) {
+            islands[islandId] = ExploreIsland(grid, islandId, pos);
+            ++islandId;
+        }
+
+        switch (islands.Count) {
+            case 0:
+                // can only toggle one cell
+                return 1;
+            case 1:
+                --islandId;
+                if (islands[islandId] < grid.Length * grid[0].Length) {
+                    // doesn't cover whole grid, can extend by one cell
+                    ++islands[islandId];
+                }
+
+                return islands[islandId];
+        }
+
+        return Coords(grid)
+            .Where(pos => grid[pos.y][pos.x] == 0)
+            .Max(pos => 1 + DIRS
+                .Select(d => Add(pos, d))
+                .Where(pos => InBounds(grid, pos) && grid[pos.y][pos.x] > 1)
+                .Select(pos => grid[pos.y][pos.x])
+                .Distinct()
+                .Sum(id => islands[id])
+            );
+    }
+}