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 Rust (1.15.1) (judge verdict: Accepted)

use std::cmp::{max, min};
use std::collections::{HashMap, HashSet, VecDeque};
use std::io::{stdin, Read};
use std::str::FromStr;

// https://qiita.com/tubo28/items/e6076e9040da57368845
fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("failed to read char") as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().expect("failed to parse token")
}

#[allow(dead_code)]
fn main() {
    let h = read::<usize>();
    let w = read::<usize>();
    let mut a = (0..h)
        .map(|_| {
            read::<String>()
                .chars()
                .map(|c| if c == '#' { 0 } else { -1 })
                .collect::<Vec<_>>()
        })
        .collect::<Vec<_>>();

    let mut q = VecDeque::new();
    for i in 0..h {
        for j in 0..w {
            if a[i][j] == 0 {
                q.push_back((i, j, 0));
                a[i][j] = -1;
            }
        }
    }
    let mut s = a;
    while let Some((i, j, c)) = q.pop_front() {
        if s[i][j] >= 0 {
            continue;
        }
        s[i][j] = c;
        if i > 0 {
            q.push_back((i - 1, j, c + 1))
        }
        if j > 0 {
            q.push_back((i, j - 1, c + 1))
        }
        if i < h - 1 {
            q.push_back((i + 1, j, c + 1))
        }
        if j < w - 1 {
            q.push_back((i, j + 1, c + 1))
        }
    }
    let mut ans = 0;
    for i in 0..h {
        for j in 0..w {
            ans = max(ans, s[i][j]);
        }
    }

    println!("{}", ans);
}