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.util.*

fun main(args: Array<String>) {
    val (h, w) = readLine()!!.split(" ").map { it.toInt() }
    val a = (0 until h).map { readLine()!!.toMutableList() }
    val q = LinkedList<Triple<Int, Int, Int>>().apply {
        a.forEachIndexed { i, ai -> ai.forEachIndexed { j, c -> if (c == '#') add(Triple(i, j, 0)) } }
    }
    var cnt=0
    while (q.isNotEmpty()) {
        val (i, j, n) = q.first()
        q.removeAt(0)
        if (i-1 in 0 until h && a[i-1][j] == '.') {
            a[i-1][j] = '#'
            q += Triple(i-1, j, n+1)
        }
        if (i+1 in 0 until h && a[i+1][j] == '.') {
            a[i+1][j] = '#'
            q += Triple(i+1, j, n+1)
        }
        if (j-1 in 0 until w && a[i][j-1] == '.') {
            a[i][j-1] = '#'
            q += Triple(i, j-1, n+1)
        }
        if (j+1 in 0 until w && a[i][j+1] == '.') {
            a[i][j+1] = '#'
            q += Triple(i, j+1, n+1)
        }
        if (cnt < n) cnt = n
    }
    println(cnt)
}