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: Time Limit Exceeded)

using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Runtime.CompilerServices;
using System.Text;
using static Template;
using static System.Console;
using static System.Convert;
using static System.Math;
using Pi = Pair<int, int>;
using Graph;
//using DataStructure;

class Solver
{
    public void Solve()
    {
        int h, w;
        Input.Make(out h, out w);
        var grid = Input.grid(h);
        var g = new BreadthFirstSearch<V>(h * w);
        var list = new List<V>();
        V.wid = w;
        for(var i=0;i<h;i++)
            for(var j = 0; j < w; j++)
            {
                if (grid[i][j] == '#')
                    list.Add(new V(i, j));
                for(var k = 0; k < 4; k++)
                {
                    int hh = gh[k] + i, ww = gw[k] + j;
                    if (Inside(hh, ww, h, w))
                        g.AddEdge(new V(i, j), new V(hh, ww));
                }
            }
        WriteLine(g.Execute(list.ToArray()).Max());
    }

    int[] gh = new[] { 0, 0, 1, -1 }, gw = new[] { 1, -1, 0, 0 };
    public static bool Inside(int h, int w, int hh, int ww)
            => 0 <= h && h < hh && 0 <= w && w < ww;
}
struct V : IVertex
{
    public static int wid;
    public int h, w;
    public V(int h,int w) { this.h = h;this.w = w; }
    public int Id { get { return h * wid + w; } }
}

namespace Graph
{
    #region テンプレート      
    public class Graph<E> where E : Edge
    {
        public List<E>[] Edges { get; set; }
        public int Count { get; }
        public Graph(int count)
        {
            Count = count;
            Edges = Create(count, () => new List<E>());
        }
    }
    public class Directed<E> : Graph<E> where E : Edge
    { public Directed(int count) : base(count) { } }
    public class Undirected<E> : Graph<E> where E : Edge
    { public Undirected(int count) : base(count) { } }
    public interface IWeight<T> where T : IComparable<T> { T Weight { get; set; } }
    public interface INNegWeight<T> : IWeight<T> where T : IComparable<T> { }
    public interface IVertex
    {
        int Id { get; }
    }
    public class Edge
    {
        public int From { get; set; }
        public int To { get; set; }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public Edge(int from, int to)
        { From = from; To = to; }
    }
    #endregion
}
namespace Graph
{
    #region 幅優先探索
    public class BreadthFirstSearch
    {
        private Graph<BfsEdge> g;
        public BreadthFirstSearch(int count)
        { g = new Graph<BfsEdge>(count); }
        public void AddEdge(int from, int to)
            => g.Edges[from].Add(new BfsEdge(from, to));
        /// <summary>
        /// O(V+E)
        /// </summary>
        /// <param name="st"></param>
        /// <returns></returns>
        public int[] Execute(int[] st)
        {
            var dist = Create(g.Count, () => -1);
            var que = new Queue<int>();
            foreach (var f in st)
            {
                que.Enqueue(f);
                dist[f] = 0;
            }
            while (que.Any())
            {
                var p = que.Dequeue();
                foreach (var e in g.Edges[p])
                    if (dist[e.To] == -1)
                    {
                        dist[e.To] = dist[p] + 1;
                        que.Enqueue(e.To);
                    }
            }
            return dist;
        }

        public class BfsEdge : Edge
        {
            public BfsEdge(int from, int to) : base(from, to)
            { }
        }
    }
    public class BreadthFirstSearch<V> where V : IVertex
    {
        private BreadthFirstSearch g;
        public BreadthFirstSearch(int count)
        { g = new BreadthFirstSearch(count);}
        public void AddEdge(V from, V to)
            => g.AddEdge(from.Id, to.Id);
        public int[] Execute(V[] st)
            => g.Execute(st.Select(a=>a.Id).ToArray());
    }
    #endregion
}

public class Template
{
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static bool chmin<T>(ref T num, T val) where T : IComparable<T>
    { if (num.CompareTo(val) == 1) { num = val; return true; } return false; }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static bool chmax<T>(ref T num, T val) where T : IComparable<T>
    { if (num.CompareTo(val) == -1) { num = val; return true; } return false; }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void swap<T>(ref T v1, ref T v2)
    { var t = v2; v2 = v1; v1 = t; }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static T[] Create<T>(int n, Func<T> f)
        => Enumerable.Repeat(0, n).Select(_ => f()).ToArray();
    public static void Fail() => Fail("No");
    public static void Fail<T>(T s) { WriteLine(s); Console.Out.Close(); Environment.Exit(0); }
    static void Main(string[] args)
    {
        var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
        Console.SetOut(sw);
        var p = new Solver();
        for (var i = 1; i > 0; --i)
            p.Solve();
        Console.Out.Flush();
    }
}

public class Input
{
    public static string read => ReadLine().Trim();
    public static int[] ar => read.Split(' ').Select(int.Parse).ToArray();
    public static int num => ToInt32(read);
    public static long[] arL => read.Split(' ').Select(long.Parse).ToArray();
    public static long numL => ToInt64(read);
    public static char[][] grid(int h)
        => Create(h, () => read.ToCharArray());
    public static int[] ar1D(int n)
        => Create(n, () => num);
    public static long[] arL1D(int n)
        => Create(n, () => numL);
    public static string[] strs(int n)
        => Create(n, () => read);
    public static int[][] ar2D(int n)
        => Create(n, () => ar);
    public static long[][] arL2D(int n)
        => Create(n, () => arL);
    public static List<T>[] edge<T>(int n)
        => Create(n, () => new List<T>());
    public static void Make<T1, T2>(out T1 v1, out T2 v2)
    {
        v1 = Next<T1>();
        v2 = Next<T2>();
    }
    public static void Make<T1, T2, T3>(out T1 v1, out T2 v2, out T3 v3)
    {
        Make(out v1, out v2);
        v3 = Next<T3>();
    }
    public static void Make<T1, T2, T3, T4>(out T1 v1, out T2 v2, out T3 v3, out T4 v4)
    {
        Make(out v1, out v2, out v3);
        v4 = Next<T4>();
    }
    public static void Make<T1, T2, T3, T4, T5>(out T1 v1, out T2 v2, out T3 v3, out T4 v4, out T5 v5)
    {
        Make(out v1, out v2, out v3, out v4);
        v5 = Next<T5>();
    }
    public static void Make<T1, T2, T3, T4, T5, T6>(out T1 v1, out T2 v2, out T3 v3, out T4 v4, out T5 v5, out T6 v6)
    {
        Make(out v1, out v2, out v3, out v4, out v5);
        v6 = Next<T6>();
    }
    static Input()
    {
        sc = new Queue<string>();
        dic = new Dictionary<Type, Func<string, object>>();
        dic[typeof(int)] = s => int.Parse(s);
        dic[typeof(long)] = s => long.Parse(s);
        dic[typeof(char)] = s => char.Parse(s);
        dic[typeof(double)] = s => double.Parse(s);
        dic[typeof(uint)] = s => uint.Parse(s);
        dic[typeof(ulong)] = s => ulong.Parse(s);
        dic[typeof(string)] = s => s;
    }
    private static Dictionary<Type, Func<string, object>> dic;
    private static Queue<string> sc;
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static T Next<T>() { if (sc.Count == 0) foreach (var item in read.Split(' ')) sc.Enqueue(item); return (T)dic[typeof(T)](sc.Dequeue()); }
    public const int MOD = 1000000007;
}

public class Pair<T1, T2> : IComparable<Pair<T1, T2>>
{
    public T1 v1 { get; set; }
    public T2 v2 { get; set; }
    public Pair() { v1 = Input.Next<T1>(); v2 = Input.Next<T2>(); }
    public Pair(T1 v1, T2 v2)
    { this.v1 = v1; this.v2 = v2; }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public int CompareTo(Pair<T1, T2> p)
    {
        var c = Comparer<T1>.Default.Compare(v1, p.v1);
        if (c == 0)
            c = Comparer<T2>.Default.Compare(v2, p.v2);
        return c;
    }
    public override string ToString()
        => $"{v1.ToString()} {v2.ToString()}";
}

public class Pair<T1, T2, T3> : Pair<T1, T2>, IComparable<Pair<T1, T2, T3>>
{
    public T3 v3 { get; set; }
    public Pair() : base() { v3 = Input.Next<T3>(); }
    public Pair(T1 v1, T2 v2, T3 v3) : base(v1, v2)
    { this.v3 = v3; }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public int CompareTo(Pair<T1, T2, T3> p)
    {
        var c = base.CompareTo(p);
        if (c == 0)
            c = Comparer<T3>.Default.Compare(v3, p.v3);
        return c;
    }
    public override string ToString()
        => $"{base.ToString()} {v3.ToString()}";
}