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 Go (1.6) (judge verdict: Accepted)

package main

import (
	"bufio"
	"fmt"
	"os"
)

type YX struct {
	y int
	x int
}

func main() {
	var h, w int
	fmt.Scan(&h, &w)
	var a = make([][]byte, h)
	var d = make([][]int, h)
	var reader = bufio.NewReaderSize(os.Stdin, 1024)
    var ql = make([]YX, 0)
    var ans = 0

	for y := 0; y < h; y++ {
		l, _, _ := reader.ReadLine()
		a[y] = make([]byte, w)
		d[y] = make([]int, w)
		copy(a[y], l)
		for x := 0; x < w; x++ {
			if a[y][x] == '.' {
				d[y][x] = -1
			} else {
				ql = append(ql, YX{y, x})
			}
		}
	}

	for len(ql) > 0 {
		var newql = make([]YX, 0)
		for _, q := range ql {
			el := []bool{false, false, false, false}
			if q.y > 0 {
				el[0] = true
			}
			if q.y < h-1 {
				el[2] = true
			}
			if q.x > 0 {
				el[3] = true
			}
			if q.x < w-1 {
				el[1] = true
            }
            ans = d[q.y][q.x]
			for i, e := range el {
				if e {
					switch i {
					case 0:
						y, x := q.y-1, q.x
						if d[y][x] == -1 {
							d[y][x] = d[q.y][q.x] + 1
                            newql = append(newql, YX{y, x})
						}
					case 1:
						y, x := q.y, q.x+1
						if d[y][x] == -1 {
							d[y][x] = d[q.y][q.x] + 1
							newql = append(newql, YX{y, x})
						}
					case 2:
						y, x := q.y+1, q.x
						if d[y][x] == -1 {
							d[y][x] = d[q.y][q.x] + 1
							newql = append(newql, YX{y, x})
						}
					case 3:
						y, x := q.y, q.x-1
						if d[y][x] == -1 {
							d[y][x] = d[q.y][q.x] + 1
							newql = append(newql, YX{y, x})
						}
					}
				}
			}
		}
		ql = newql
	}

	fmt.Println(ans)
}