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 (Python):
from collections import deque

H, W = map(int, input().split())
que = deque()
A = [input() for i in range(H)]

dist = [[-1]*W for i in range(H)]
dd = ((-1, 0), (0, -1), (1, 0), (0, 1))
for i in range(H):
    for j in range(W):
        if A[i][j] == '
            dist[i][j] = 0
            que.append((i, j))

d = 0
while que:
    y, x = que.popleft()
    d = dist[y][x]
    for dx, dy in dd:
        nx = x + dx; ny = y + dy
        if not 0 <= nx < W or not 0 <= ny < H or dist[ny][nx] != -1:
            continue
        dist[ny][nx] = d+1
        que.append((ny, nx))
print(d)