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 (
	"bufio"
	"fmt"
	"log"
	"math"
	"os"
	"strconv"
)

func main() {
	H := nextInt()
	W := nextInt()
	A := make([][]byte, H)
	for i := 0; i < H; i++ {
		A[i] = nextBytes()
		if len(A[i]) != W {
			panic("W")
		}
	}
	q := make([]Pair, 0)
	next := make([]Pair, 0)
	for i := 0; i < H; i++ {
		for j := 0; j < W; j++ {
			if A[i][j] == '#' {
				next = append(next, Pair{i, j})
			}
		}
	}
	ans := 0
	for len(next) != 0 {
		q = make([]Pair, 0)
		for _, n := range next {
			q = append(q, n)
		}
		next = make([]Pair, 0)
		for len(q) != 0 {
			p := q[len(q)-1]
			q = q[:len(q)-1]
			for d := 0; d < 4; d++ {
				ny := p.a + dy[d]
				nx := p.b + dx[d]
				if ny < 0 || ny >= H || nx < 0 || nx >= W {
					continue
				}
				if A[ny][nx] == '.' {
					A[ny][nx] = '#'
					next = append(next, Pair{ny, nx})
				}
			}
		}
		ans++
		// log.Println(next)
		// for i := 0; i < H; i++ {
		// 	log.Println(string(A[i]))
		// }
	}
	fmt.Println(ans - 1)
}

var dy = []int{0, 1, 0, -1}
var dx = []int{1, 0, -1, 0}
var inf = int(1e9)

func pow(a, b int) int {
	return int(math.Pow(float64(a), float64(b)))
}

func lcm(a, b int) int {
	return a * b / gcd(a, b)
}
func gcd(a, b int) int {
	if a < b {
		a, b = b, a
	}
	for b != 0 {
		a, b = b, a%b
	}
	return a
}
func max(a ...int) int {
	r := a[0]
	for i := 0; i < len(a); i++ {
		if r < a[i] {
			r = a[i]
		}
	}
	return r
}
func min(a ...int) int {
	r := a[0]
	for i := 0; i < len(a); i++ {
		if r > a[i] {
			r = a[i]
		}
	}
	return r
}
func sum(a []int) (r int) {
	for i := range a {
		r += a[i]
	}
	return r
}
func abs(a int) int {
	if a < 0 {
		return -a
	}
	return a
}

type Pair struct {
	a, b int
}

type pairs []Pair

func (p pairs) Len() int {
	return len(p)
}
func (p pairs) Swap(i, j int) {
	p[i], p[j] = p[j], p[i]
}

func (p pairs) Less(i, j int) bool {
	if p[i].a == p[j].a {
		return p[i].b < p[j].b
	}
	return p[i].a < p[j].a
}

var nextReader func() []byte

func init() {
	nextReader = newScanner()
	log.SetFlags(log.Lshortfile)
}

func newScanner() func() []byte {
	r := bufio.NewScanner(os.Stdin)
	r.Split(bufio.ScanWords)
	return func() []byte {
		r.Scan()
		return r.Bytes()
	}
}
func nextBytes() []byte {
	return nextReader()
}

func nextString() string {
	return string(nextReader())
}
func nextInt64() int64 {
	v, _ := strconv.ParseInt(string(nextReader()), 10, 64)
	return v
}
func nextInt() int {
	v, _ := strconv.Atoi(string(nextReader()))
	return v
}
func nextInts(n int) []int {
	r := make([]int, n)
	for i := 0; i < n; i++ {
		r[i] = nextInt()
	}
	return r
}
func nextFloat64() float64 {
	f, _ := strconv.ParseFloat(string(nextReader()), 64)
	return f
}