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: Time Limit Exceeded)

import java.io.PrintWriter

val pw = PrintWriter(System.out)

fun main(args : Array<String>) {
    func()
    pw.flush()
}

fun func() {
    val (h, w) = listOfInt()
    var a = emptyList<String>().toMutableList()
    for (i in 0 until h){
        a.add(next())
    }

    var origin = emptyList<List<Int>>().toMutableList()

    var max = 0

    for (i in 0 until h){
        for (j in 0 until w){
            if (a[i][j] == '#'){
                if ((i != 0 && a[i-1][j] == '.') || (j != 0 && a[i][j-1] == '.') ||
                    (i != h-1 && a[i+1][j] == '.' || (j != w-1 && a[i][j+1] == '.')))
                    origin.add(listOf(i,j))
            }
        }
    }

    var temp = 0
    for (i in 0 until h){
        for (j in 0 until w){
            if (a[i][j] == '.'){
                temp = h+w
                origin.forEach {
                    var x = it[0]-i
                    var y = it[1]-j

                    if (x < 0) x *= -1
                    if (y < 0) y *= -1
                    var distance = x+y
                    if (temp > distance) temp = distance
                }
                if (max < temp) max = temp
            }
        }
    }

    println(max)
}

// 入力取得
fun next() = readLine()!!
fun listOfString() = next().split(" ")
fun listOfInt() =listOfString().map { it.toInt() }