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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Linq;
using System.Collections.Generic;

namespace Competition
{
    class Program
    {
        static int[] dx = { 1, 0, -1, 0 };
        static int[] dy = { 0, 1, 0, -1 };
        static int H, W;
        static int sx, sy, gx, gy;
        static string[] field = new string[1010];
        static long[,] d = new long[1010,1010];

        public static void bfs()
        {
            Queue<Tuple<int, int>> que = new Queue<Tuple<int, int>>();
            for(int i = 0; i < H; i++)
            {
                for(int j = 0; j < W; j++)
                {
                    d[i, j] = 1 << 29;
                }
            }
            for (int i = 0; i < H; i++)
            {
                for (int j = 0; j < W; j++)
                {
                    if (field[i][j] == '#')
                    {
                        que.Enqueue(new Tuple<int, int>(i, j));
                        d[i, j] = 0;
                    }
                }
            }

            while (que.Count() > 0)
            {
                var p = que.Dequeue();
                for(int i = 0; i < 4; i++)
                {
                    var ny = p.Item1 + dy[i];
                    var nx = p.Item2 + dx[i];

                    if (nx >= 0 && nx < W && ny >= 0 && ny < H && field[ny][nx] != '#' && d[ny,nx] == 1 << 29)
                    {
                        que.Enqueue(new Tuple<int, int>(ny,nx));
                        d[ny,nx] = d[p.Item1, p.Item2] + 1;
                    }
                }
            }
        }

        static void Main(string[] args)
        {
            var hw = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
            H = hw[0]; W = hw[1];

            for (int i = 0; i < H; i++)
            {
                field[i] = Console.ReadLine();
            }
            bfs();
            long res = 0;
            for (int i = 0; i < H; i++)
            {
                for (int j = 0; j < W; j++)
                {
                    if (d[i, j] > res) res = d[i, j];
                }
            }
            Console.WriteLine(res);
        }
    }
}