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: Wrong Answer)

package main

import (
	"fmt"
)

var H, W int
var s string
var max int
var dir = [4][2]int{{1, 0}, {0, 1}, {-1, 0}, {0, -1}}

func Min(x, y int) int {
	if x < y {
		return x
	}
	return y
}

func Max(x, y int) int {
	if x > y {
		return x
	}
	return y
}

func Abs(x int) int {
	if x < 0 {
		return -x
	}
	return x
}

func contains(s [][2]int, e [2]int) bool {
	for _, v := range s {
		if e == v {
			return true
		}
	}
	return false
}

func bfs(h, w int, field [][]bool) int {
	min := 1
	now := [][2]int{{h, w}}
	var nextNow [][2]int
	visited := [][2]int{{h, w}}
L:
	for {
		for _, n := range now {
			for _, d := range dir {
				next := [2]int{n[0] + d[0], n[1] + d[1]}
				if 0 <= next[0] && next[0] <= H-1 && 0 <= next[1] && next[1] <= W-1 {
					if !contains(visited, next) {
						if field[next[0]][next[1]] {
							break L
						}
						visited = append(visited, next)
						nextNow = append(nextNow, next)
					}
				}
			}
		}
		now = nextNow
		nextNow = [][2]int{{}}
		min++
	}
	return min
}

func main() {
	fmt.Scan(&H, &W)
	var field [][]bool
	field = make([][]bool, H)
	for h := 0; h < H; h++ {
		field[h] = make([]bool, W)
		fmt.Scan(&s)
		for w := 0; w < W; w++ {
			if s[w] == '#' {
				field[h][w] = true
			}
		}
	}
	for h := 0; h < H; h++ {
		for w := 0; w < W; w++ {
			if !field[h][w] {
				max = Max(max, bfs(h, w, field))
			}
		}
	}
	fmt.Println(max)
}