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 LuaJIT (2.0.4) (judge verdict: Accepted)

local function str2tbl(s)
    local t = {string.byte(s, 1, #s)}
    for i=1, #t do
        t[i] = string.char(t[i])
    end
    return t
end

-- http://www.lua.org/pil/11.4.html
local function List_new()
    local tbl = {}
    local first = 1
    local last = 0
    local function count()
        return last - first + 1
    end
    local function pushright(value)
        local new_last = last + 1
        last = new_last
        tbl[last] = value
    end
    local function popleft(value)
        --if first > last then error("list is empty") end
        local value = tbl[first]
        tbl[first] = nil
        first = first + 1
        return value
    end
    return pushright, popleft, count
end

------
local H, W = io.read("*n", "*n", "*l")
local grid = {}
for i=1, H do
    grid[i] = str2tbl(io.read("*l"))
end
--[[
for i=1, H do
    for j=1, W do
        print(i,j, grid[i][j])
    end
end
]]
local loops = 0
local count = 0
local blacks_pushright, blacks_popleft, blacks_count = List_new()
local blacks_used = {}
for i=1, 1000*1000 + 1000 do
    blacks_used[i] = false
end
local function not_blacks_used(i, j)
    if not blacks_used[i*1000+j] then
        blacks_used[i*1000+j] = true
        return true
    end
    return false
end
for i=1, H do
    for j=1, W do
        local c = grid[i][j]
        loops = loops + 1
        if c == '#' then
            if not_blacks_used(i, j) then
                blacks_pushright({i, j})
            end
        end
    end
end

local cur_count = 0
while blacks_count() > 0 do
    for bc=blacks_count(),1,-1 do
        local t = blacks_popleft()
        local i, j = t[1], t[2]
        for dyx=0,3 do
            local y01, x01 = dyx/2 - dyx/2%1, dyx%2
            local dy = y01 * (-1 + 2 * x01)
            local dx = (1 - y01) * (-1 + 2 * x01)
            --print(dyx,y01,x01,dy,dx)
            local x = i + dx
            local y = j + dy
            if x >= 1 and x <= H and y >= 1 and y <= W then
                if not_blacks_used(x, y) then
                    blacks_pushright({x, y})
                end
            end
        end
    end
    count = count + 1
end
print(count-1)