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: Accepted)

import java.util.*

fun main(args: Array<String>) {
    agc033a()
}

private data class Coordinate(val y: Int, val x: Int)

fun agc033a() {
    val (h, w) = readLine()!!.split(' ').map { it.toInt() }
    val isBlack = (1..h).map { readLine()!!.map { it == '#' }.toBooleanArray() }.toTypedArray()

    val blackIndices = mutableListOf<Coordinate>()
    val distantList = Array(h) { IntArray(w) { 99999 } }

    for (y in 0 until h) {
        for (x in 0 until w) {
            if (isBlack[y][x]) {
                blackIndices.add(Coordinate(y, x))
                distantList[y][x] = 0
            }
        }
    }

    val queue = ArrayDeque(blackIndices)
    val direction = listOf(-1 to 0, 1 to 0, 0 to -1, 0 to 1)
    var answer = 0

    while (queue.isNotEmpty()) {
        val (y, x) = queue.poll()

        val dist = distantList[y][x]
        answer = Math.max(answer, dist)

        direction.forEach {
            val nextY = y + it.first
            val nextX = x + it.second
            if (nextY in 0 until h && nextX in 0 until w && !isBlack[nextY][nextX]) {
                isBlack[nextY][nextX] = true
                queue.add(Coordinate(nextY, nextX))
                distantList[nextY][nextX] = Math.min(distantList[nextY][nextX], dist + 1)
            }
        }
    }

    println(answer)
}