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 Scala (2.11.7) (judge verdict: Time Limit Exceeded)

object Main {

  def main(args: Array[String]): Unit = {
    val Array(h, w) = scala.io.StdIn.readLine().split(" ").map(_.toInt)
    var A = Array.fill[Boolean](h+2, w+2)(true)

    var wc = 0
    (1 to h) foreach { i =>
      scala.io.StdIn.readLine().zipWithIndex.foreach { case (c, j) =>
        A(i)(j+1) = if (c == '.') {
          wc += 1
          true
        } else false
      }
    }

    var c = 0
    while (wc > 0) {
      val cloneA = A.map(_.clone()).clone()
      for {
        i <- 1 to h
        j <- 1 to w
        if A(i)(j)
        if !A(i)(j-1) || !A(i)(j+1) || !A(i-1)(j) || !A(i+1)(j)
      } {
        wc -= 1
        cloneA(i)(j) = false
      }
      c += 1
      A = cloneA
    }

    println(c)
  }

}