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: Wrong Answer)

function returnNextStr(input, index, L, W) {
  var result = input.slice();
  if (index - 1 > -1) {
    result[index - 1] = '#';
  }
  if (index + 1 < L) {
    result[index + 1] = '#';
  }
  if (index - W > -1) {
    result[index - W] = '#';
  }
  if (index + W < L) {
    result[index + W] = '#';
  }

  return result;
}

function Main(input) {
  // 1行目がinput[0], 2行目がinput[1], …に入る
  input = input.split("\n");

  var result = 0;

  var tmp = input[0].split(" ");
  var H = parseInt(tmp[0], 10);
  var W = parseInt(tmp[1], 10);
  var L = H * W;
  var resltStr = '';
  for (var i = 0; i < H; i++) {
    resltStr += input[i + 1];
  }
  var resltArr = resltStr.split('');
  var tmpArr = resltArr.slice();
  while (resltArr.indexOf('.') != -1 && resltArr.indexOf('#') != -1) {
    for (var i = 0; i < L; i++) {
      if (resltArr[i] == '#') {
        tmpArr = returnNextStr(tmpArr, i, L, W);
      }
    }
    resltArr = tmpArr.slice();
    result++;
  }

  //出力
  console.log(result);
}