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.*
import java.math.*
import java.lang.Math.*


private val readString: ()->String = {readLine()!!}
private val readInt: ()->Int = { readLine()!!.toInt() }
private val readLong: ()->Long = { readLine()!!.toLong() }
private val readListInt: ()->List<Int> = { readLine()!!.split(' ').map(String::toInt) }
private val readListLong: ()->List<Long> = { readLine()!!.split(' ').map(String::toLong) }
private val errPrintln: (String)->Unit = { msg -> System.err.println(msg) }


data class Position(var x: Int, var y: Int)

fun solveA(G: Array<String>): Int {
    val H = G.size
    val W = G[0].length
    val INF = Int.MAX_VALUE / 2
    val step = Array(H) { IntArray(W) }
    val starts = ArrayList<Position>()
    for (i in 0 until H) {
        for (j in 0 until W) {
            step[i][j] = INF
            if (G[i][j] == '#') {
                starts.add(Position(i, j))
            }
        }
    }
    val dx = intArrayOf(0, 1, 0, -1)
    val dy = intArrayOf(1, 0, -1, 0)
    val que = ArrayDeque<Position>()
    for (i in starts.indices) {
        que.addLast(starts[i])
        step[starts[i].x][starts[i].y] = 0
    }
    var res = 0
    while (!que.isEmpty()) {
        val pos = que.removeFirst()
        val x = pos.x
        val y = pos.y
        for (i in dx.indices) {
            val nx = x + dx[i]
            val ny = y + dy[i]
            if (nx < 0 || H <= nx || ny < 0 || W <= ny) {
                continue
            }
            if (step[nx][ny] > step[x][y] + 1) {
                step[nx][ny] = step[x][y] + 1
                que.addLast(Position(nx, ny))
                res = max(res, step[nx][ny])
            }
        }
    }
    return res
}


fun main(args: Array<String>) {
    val (H, W) = readListInt()
    val A = Array<String>(H){""}
    for (i in 0 until H) {
        A[i] = readString()
    }

    val ans = solveA(A)

    println(ans)
}