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 JavaScript (node.js v5.12) (judge verdict: Runtime Error)

"use strict"

function main(arg) {
    arg = arg.trim().split("\n")
    arg.shift()
    var H = arg.length
    var W = arg[0].length
    var m1 = []
    for (var i = 0; i < arg.length; i++) {
        m1.push(arg[i].split(''))
    }
    var i = 0
    var slist = []
    for (var y = 0; y < H; y++) for (var x = 0; x < W; x++) {
        if (m1[y][x] === '#') {
            slist.push([y, x])
        }
    }
    var tmp = []
    for (var y = 0; y < H; y++) for (var x = 0; x < W; x++) {
        if (m1[y][x] === '.') {
            var minD = Number.MAX_SAFE_INTEGER
            for (var i = 0; i < slist.length; i++) {
                var _y = slist[i][0]
                var _x = slist[i][1]
                var d = Math.abs(_y - y) + Math.abs(_x - x)
                if (d < minD) {
                    minD = d
                }
            }
            tmp.push(minD)
        }
    }
    console.log(Math.max(...tmp))
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));