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 dirs = [][]int{{1, 0}, {0, 1}, {-1, 0}, {0, -1}}

func check(field [][]bool) bool {
	for h := 0; h < H; h++ {
		for _, x := range field[h] {
			if !x {
				return true
			}
		}
	}
	return false
}

func main() {
	fmt.Scan(&H, &W)
	var field, new_field [][]bool
	field = make([][]bool, H)
	new_field = make([][]bool, H)
	for h := 0; h < H; h++ {
		field[h] = make([]bool, W)
		new_field[h] = make([]bool, W)
		fmt.Scan(&s)
		for w := 0; w < W; w++ {		
			if s[w] == '#' {
				field[h][w] = true
				new_field[h][w] = true
			}
		}
	}
	count := 0
	for check(field) {
		for h, f := range field {
			for w, ff := range f {
				if ff {
					for _, d := range dirs {
						t := h + d[0]
						y := w + d[1]
						if 0 <= t && t <= H-1 && 0 <= y && y <= W-1 {
							new_field[t][y] = true
						}
					}
				}
			}
		}
		count++
		copy(new_field, field)
	}
	fmt.Println(count)
}