Buggy python submission (verdict: Runtime Error):
import copy
import itertools
    
h,w = (int(x) for x in input().split())
ab = []
step = []
count = 0
for i in range(h):
    a = list(input())
    ab.append(a+['#'])
    for j in range(w):
        if a[j] == '#':
            step.append([i,j])

ab.append(list('#'*w))           

dp = [[0 for a in range(w)] for b in range(h)]
 
while len(step) > 0:
    y, x = step.pop(0)
    if ab[y+1][x] == '.' and dp[y+1][x] == 0 :
        step.append([y+1, x])
        dp[y+1][x] = dp[y][x] + 1
    if ab[y][x+1] == '.' and dp[y][x+1] == 0 :
        step.append([y, x+1])
        dp[y][x+1] = dp[y][x] + 1
    if ab[y-1][x] == '.' and dp[y-1][x] == 0 :
        step.append([y-1, x])
        dp[y-1][x] = dp[y][x] + 1
    if ab[y][x-1] == '.' and dp[y][x-1] == 0 :
        step.append([y, x-1])
        dp[y][x-1] = dp[y][x] + 1
    print(step)

#mx = max(list(itertools.chain.from_iterable(dp)))
 
print(mx[0][0])

Fixed submission:
from collections import deque
 
H, W = [int(x) for x in input().split()]
cell = [list(input()) for i in range(H)]
 
painted = deque()
for h in range(H):
    for w in range(W):
        if cell[h][w] == '#':
            painted.append([h, w, 0])
 
ans = 0
while len(painted) > 0:
    blackcell = painted.popleft()
    for dy, dx in [[1, 0], [0, 1], [-1, 0], [0, -1]]:
        nh = blackcell[0] + dy
        nw = blackcell[1] + dx
        d = blackcell[2]
        if 0 <= nh < H and 0 <= nw < W and cell[nh][nw] == '.':
            cell[nh][nw] = '#'
            painted.append([nh, nw, d + 1])
        ans = d
 
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import copy
import itertools
    
h,w = (int(x) for x in input().split())
ab = []
step = []
count = 0
for i in range(h):
    a = list(input())
    ab.append(a+['#'])
    for j in range(w):
        if a[j] == '#':
            step.append([i,j])

ab.append(list('#'*w))           

dp = [[0 for a in range(w)] for b in range(h)]
 
while len(step) > 0:
    y, x = step.pop(0)
    if ab[y+1][x] == '.' and dp[y+1][x] == 0 :
        step.append([y+1, x])
        dp[y+1][x] = dp[y][x] + 1
    if ab[y][x+1] == '.' and dp[y][x+1] == 0 :
        step.append([y, x+1])
        dp[y][x+1] = dp[y][x] + 1
    if ab[y-1][x] == '.' and dp[y-1][x] == 0 :
        step.append([y-1, x])
        dp[y-1][x] = dp[y][x] + 1
    if ab[y][x-1] == '.' and dp[y][x-1] == 0 :
        step.append([y, x-1])
        dp[y][x-1] = dp[y][x] + 1

mx = max(list(itertools.chain.from_iterable(dp)))
 
print(mx)

Fixed submission:
from collections import deque
 
H, W = [int(x) for x in input().split()]
cell = [list(input()) for i in range(H)]
 
painted = deque()
for h in range(H):
    for w in range(W):
        if cell[h][w] == '#':
            painted.append([h, w, 0])
 
ans = 0
while len(painted) > 0:
    blackcell = painted.popleft()
    for dy, dx in [[1, 0], [0, 1], [-1, 0], [0, -1]]:
        nh = blackcell[0] + dy
        nw = blackcell[1] + dx
        d = blackcell[2]
        if 0 <= nh < H and 0 <= nw < W and cell[nh][nw] == '.':
            cell[nh][nw] = '#'
            painted.append([nh, nw, d + 1])
        ans = d
 
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import copy
import itertools
    
h,w = (int(x) for x in input().split())
ab = []
step = []
count = 0
for i in range(h):
    a = list(input())
    ab.append(a+['#'])
    for j in range(w):
        if a[j] == '#':
            step.append([i,j])

ab.append(list('#'*w))           
memo_step = copy.deepcopy(step)

dp = [[0 for a in range(w)] for b in range(h)]
 
while len(step) > 0:
    y, x = step.pop(0)
    if ab[y+1][x] == '.' and dp[y+1][x] == 0 :
        step.append([y+1, x])
        dp[y+1][x] = dp[y][x] + 1
    if ab[y][x+1] == '.' and dp[y][x+1] == 0 :
        step.append([y, x+1])
        dp[y][x+1] = dp[y][x] + 1
    if ab[y-1][x] == '.' and dp[y-1][x] == 0 :
        step.append([y-1, x])
        dp[y-1][x] = dp[y][x] + 1
    if ab[y][x-1] == '.' and dp[y][x-1] == 0 :
        step.append([y, x-1])
        dp[y][x-1] = dp[y][x] + 1

for i,j in memo_step:
    dp[i][j] = 0

mx = max(list(itertools.chain.from_iterable(dp)))
 
print(mx)


Fixed submission:
from collections import deque
 
H, W = [int(x) for x in input().split()]
cell = [list(input()) for i in range(H)]
 
painted = deque()
for h in range(H):
    for w in range(W):
        if cell[h][w] == '#':
            painted.append([h, w, 0])
 
ans = 0
while len(painted) > 0:
    blackcell = painted.popleft()
    for dy, dx in [[1, 0], [0, 1], [-1, 0], [0, -1]]:
        nh = blackcell[0] + dy
        nw = blackcell[1] + dx
        d = blackcell[2]
        if 0 <= nh < H and 0 <= nw < W and cell[nh][nw] == '.':
            cell[nh][nw] = '#'
            painted.append([nh, nw, d + 1])
        ans = d
 
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import copy
import itertools
    
h,w = (int(x) for x in input().split())
ab = []
step = []
count = 0
for i in range(h):
    a = list(input())
    ab.append(a+['#'])
    for j in range(w):
        if a[j] == '#':
            step.append([i,j])

ab.append(list('#'*w))           
memo_step = copy.deepcopy(step)

dp = [[0 for a in range(w)] for b in range(h)]
 
while len(step) > 0:
    y, x = step.pop(0)
    if ab[y+1][x] == '.' and dp[y+1][x] == 0 and -1 < x < w and -1 < y < h :
        step.append([y+1, x])
        dp[y+1][x] = dp[y][x] + 1
    if ab[y][x+1] == '.' and dp[y][x+1] == 0 and -1 < x < w and -1 < y < h :
        step.append([y, x+1])
        dp[y][x+1] = dp[y][x] + 1
    if ab[y-1][x] == '.' and dp[y-1][x] == 0 and -1 < x < w and -1 < y < h :
        step.append([y-1, x])
        dp[y-1][x] = dp[y][x] + 1
    if ab[y][x-1] == '.' and dp[y][x-1] == 0 and -1 < x < w and -1 < y < h :
        step.append([y, x-1])
        dp[y][x-1] = dp[y][x] + 1

for i,j in memo_step:
    dp[i][j] = 0

mx = max(list(itertools.chain.from_iterable(dp)))
 
print(mx)


Fixed submission:
from collections import deque
 
H, W = [int(x) for x in input().split()]
cell = [list(input()) for i in range(H)]
 
painted = deque()
for h in range(H):
    for w in range(W):
        if cell[h][w] == '#':
            painted.append([h, w, 0])
 
ans = 0
while len(painted) > 0:
    blackcell = painted.popleft()
    for dy, dx in [[1, 0], [0, 1], [-1, 0], [0, -1]]:
        nh = blackcell[0] + dy
        nw = blackcell[1] + dx
        d = blackcell[2]
        if 0 <= nh < H and 0 <= nw < W and cell[nh][nw] == '.':
            cell[nh][nw] = '#'
            painted.append([nh, nw, d + 1])
        ans = d
 
print(ans)