kt: add «2257. Count Unguarded Cells in the Grid»

URL:	https://leetcode.com/problems/count-unguarded-cells-in-the-grid/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/count-unguarded-cells-in-the-grid.kt

@@ -0,0 +1,79 @@
+class Solution {
+    private fun <A, B> product(
+        xs: Sequence<A>,
+        ys: Sequence<B>,
+    ): Sequence<Pair<A, B>> = xs.flatMap { x -> ys.map { y -> x to y } }
+
+    private fun <A, B> product(
+        xs: Iterable<A>,
+        ys: Iterable<B>,
+    ): Sequence<Pair<A, B>> = product(xs.asSequence(), ys.asSequence())
+
+    private enum class State {
+        Unguarded,
+        Guarded,
+        Guard,
+        Wall,
+    }
+
+    private fun update(
+        grid: MutableList<MutableList<State>>,
+        y: Int,
+        x: Int,
+        guarded: Boolean,
+    ): Boolean =
+        when (grid[y][x]) {
+            State.Guard -> true
+            State.Wall -> false
+            else -> {
+                if (guarded) {
+                    grid[y][x] = State.Guarded
+                }
+
+                guarded
+            }
+        }
+
+    fun countUnguarded(
+        m: Int,
+        n: Int,
+        guards: Array<IntArray>,
+        walls: Array<IntArray>,
+    ): Int {
+        // Initialize grid
+        val grid =
+            MutableList(m) {
+                MutableList(n) { State.Unguarded }
+            }
+
+        // Place guards
+        for (guard in guards) {
+            val (y, x) = guard[0] to guard[1]
+            grid[y][x] = State.Guard
+        }
+
+        // Place walls
+        for (wall in walls) {
+            val (y, x) = wall[0] to wall[1]
+            grid[y][x] = State.Wall
+        }
+
+        // Process horizontally
+        product(0..<m, listOf(0..<n, (0..<n).reversed())).forEach { (y, direction) ->
+            direction.fold(false) { isGuardLineActive, x ->
+                update(grid, y, x, isGuardLineActive)
+            }
+        }
+
+        // Process vertically
+        product(0..<n, listOf(0..<m, (0..<m).reversed())).forEach { (x, direction) ->
+            direction.fold(false) { isGuardLineActive, y ->
+                update(grid, y, x, isGuardLineActive)
+            }
+        }
+
+        return grid.sumOf { row ->
+            row.count { it == State.Unguarded }
+        }
+    }
+}