Problem:
Score : 300 points

Problem Statement

You are given a grid of squares with H horizontal rows and W vertical columns, where each square is painted white or black.
HW characters from A_{11} to A_{HW} represent the colors of the squares.
A_{ij} is # if the square at the i-th row from the top and the j-th column from the left is black, and A_{ij} is . if that square is white.

We will repeatedly perform the following operation until all the squares are black:

Every white square that shares a side with a black square, becomes black.

Find the number of operations that will be performed.
The initial grid has at least one black square.

Constraints

1 \leq H,W \leq 1000

A_{ij} is # or ..

The given grid has at least one black square.

Input

Input is given from Standard Input in the following format:

H W
A_{11}A_{12}...A_{1W}
:
A_{H1}A_{H2}...A_{HW}

Output

Print the number of operations that will be performed.

Sample Input 1

3 3
...
.#.
...

Sample Output 1

2

After one operation, all but the corners of the grid will be black. After one more operation, all the squares will be black.

Sample Input 2

6 6
..#..#
......
#..#..
......
.#....
....#.

Sample Output 2

3

Solution in Kotlin (1.0.0) (judge verdict: Runtime Error)

package agc033

import java.util.*

fun read(): Triple<Int, Int, Array<String>> {
    val sc = Scanner(System.`in`)
    val h = sc.nextInt()
    val w = sc.nextInt()
    val a = Array<String>(h) { sc.next() }
    return Triple(w, h, a)
}

fun read2(): Triple<Int, Int, Array<String>> {
    val (h, w) = readLine()!!.split(" ").map(String::toInt)
    val a = Array(h){readLine()!!}
    return Triple (w, h, a)
}


fun solve(w: Int, h: Int, a: Array<String>): Int {


    data class Point(val x: Int, val y: Int)

    fun isWithin(p: Point): Boolean {
        return (p.x in 0 until w) && (p.y in 0 until h)
    }

    val open: Queue<Point> = ArrayDeque()
    val infty = w + h
    val dist = Array(h) { Array(w) { infty } }
    val visited = Array(h) { Array(w) { false } }
    for (y in 0 until h) {
        for (x in 0 until w) {
            if (a[y][x] == '#') {
                dist[y][x] = 0
                visited[y][x] = true
                open.add(Point(x, y))
            }
        }
    }

    var maxDist = 0
    fun update(current: Point, next: Point) {
        if (isWithin(next) && !visited[next.y][next.x]) {
            open.add(next)
            visited[next.y][next.x] = true
            dist[next.y][next.x] = dist[current.y][current.x] + 1
            maxDist = Math.max(maxDist, dist[next.y][next.x])
        }
    }
    while (open.isNotEmpty()) {
        val current = open.remove()
        update(current, Point(current.x + 1, current.y))
        update(current, Point(current.x - 1, current.y))
        update(current, Point(current.x, current.y + 1))
        update(current, Point(current.x, current.y - 1))
    }
    return maxDist
}

fun main(args: Array<String>) {
    val (w, h, a) = read2()
    println(solve(w, h, a))
}