fun <A, B> product(xs: Sequence<A>, ys: Sequence<B>): Sequence<Pair<A, B>> =
    xs.flatMap { x -> ys.map { y -> x to y } }

fun <A, B> product(xs: Iterable<A>, ys: Iterable<B>): Sequence<Pair<A, B>> =
    product(xs.asSequence(), ys.asSequence())
    
class Solution {
    fun BFS(grid: Array<IntArray>, coords: Pair<Int, Int>): Int {
        val queue = ArrayDeque<Pair<Int, Int>>()
        queue.addLast(coords)

        var size = 0
        while (!queue.isEmpty()) {
            val (y, x) = queue.removeFirst()
            if (grid[y][x] != 1) {
                continue
            }
            
            for ((dy, dx) in sequenceOf(0 to 1, 1 to 0, 0 to -1, -1 to 0)) {
                if (!(y + dy in grid.indices) || !(x + dx in grid[y + dy].indices)) {
                    continue
                } else if (grid[y + dy][x + dx] != 1) {
                    continue
                }

                queue.addLast(y + dy to x + dx)
            }
            
            // mark it as done
            grid[y][x] = 0
            size++
        }
        
        return size
    }
    
    fun maxAreaOfIsland(grid: Array<IntArray>): Int =
        product(grid.indices, grid.first().indices).map { BFS(grid, it) }.max() ?: 0
}