Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
h, w = map(int,input().split())
grid = [list(input()) for _ in range(h)]
dist = [[-1]*w for _ in range(h)]
black_cells = []
for i in range(h):
    for j in range(w):
        if grid[i][j] == '#':
            black_cells.append((i,j))
dxy = [[1,0],[-1,0],[0,1],[0,-1]]
stack = deque(black_cells)
darked = len(stack)
targetd = h*w
cnt = 0

while darked < targetd:
    next = deque([])
    while stack:
        y, x = stack.popleft()
        for dx, dy in dxy:
            X = x + dx
            Y = y + dy
            if X < 0 or X >= w or Y < 0 or Y >= h:
                continue
            if grid[Y][X] == '.':
                next.append((Y, X))
                grid[Y][X] = '#'
    stack = next
    cnt += 1
    darked += len(next)
print(cnt)


Fixed submission:
from collections import deque
 
def solve(h, w, a):
    used = [[True] * (w+2) for _ in range(h+2)]
    que = deque()
    for r in range(h):
        used[r+1][1:-1] = map(lambda _:_=="#", a[r])
        for c in range(w):
            if a[r][c] == "#":
                que.append((r+1, c+1, 0))
    neighbors = [(1,0), (0,1), (-1,0), (0,-1)]
    while que:
        r, c, k = que.popleft()
        for dr, dc in neighbors:
            nr, nc = r+dr, c+dc
            if not used[nr][nc]:
                que.append((nr, nc, k+1))
                used[nr][nc] = True
    return k
 
h, w = map(int, input().split())
a = [input() for _ in range(h)]
print(solve(h, w, a))

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque

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

# 初期の黒('#')の位置からの距離
dist = [[-1]*W for _ in range(H)]
black_cells = deque()
for h in range(H):
    for w in range(W):
        if grid[h][w] == '#':
            black_cells.append((h, w))
            dist[h][w] = 0

def bfs(black_cells, dist):
    d = 0
    while black_cells:
        # キューとして使うのでpop()ではなくpopleft()
        h, w = black_cells.popleft()
        d = dist[h][w]
        for dy, dx in ((1, 0), (0, 1), (-1, 0), (0, -1)):
            new_h = h + dy
            new_w = w + dx
            if new_h < 0 or H <= new_h or new_w < 0 or W <= new_w:
                continue
            if dist[new_h][new_w] == -1:  # セルが白('.')であるのと同義
                dist[new_h][new_w] = d+1
                black_cells.append((new_h, new_w))
    return d

d = bfs(black_cells, dist)
print(d)

Fixed submission:
from collections import deque
 
def solve(h, w, a):
    used = [[True] * (w+2) for _ in range(h+2)]
    que = deque()
    for r in range(h):
        used[r+1][1:-1] = map(lambda _:_=="#", a[r])
        for c in range(w):
            if a[r][c] == "#":
                que.append((r+1, c+1, 0))
    neighbors = [(1,0), (0,1), (-1,0), (0,-1)]
    while que:
        r, c, k = que.popleft()
        for dr, dc in neighbors:
            nr, nc = r+dr, c+dc
            if not used[nr][nc]:
                que.append((nr, nc, k+1))
                used[nr][nc] = True
    return k
 
h, w = map(int, input().split())
a = [input() for _ in range(h)]
print(solve(h, w, a))

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque

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

# 初期の黒('#')の位置からの距離
dist = [[-1]*W for _ in range(H)]
black_cells = deque()
for h in range(H):
    for w in range(W):
        if grid[h][w] == '#':
            black_cells.append((h, w))
            dist[h][w] = 0

def bfs(black_cells, dist):
    """
    BFSを用いて、全マス目を黒くする回数を求める

    Args:
        black_cells: 「開始時の黒マス」の座標が入ったキュー（複数座標あり）
        dist: 「開始時の黒マス」からの距離の入ったリスト  [[-1, -1, 0], ...]
        　　　（白マスの初期値は0）

    Returns:
        g: 操作を繰り返した数
    """
    d = 0
    while black_cells:
        # キューとして使うのでpop()ではなくpopleft()
        h, w = black_cells.popleft()
        d = dist[h][w]
        for dy, dx in ((1, 0), (0, 1), (-1, 0), (0, -1)):
            new_h = h + dy
            new_w = w + dx
            if new_h < 0 or H <= new_h or new_w < 0 or W <= new_w:
                continue
            if dist[new_h][new_w] == -1:  # セルが白('.')であるのと同義
                dist[new_h][new_w] = d+1
                black_cells.append((new_h, new_w))
    return d

d = bfs(black_cells, dist)
print(d)
        

Fixed submission:
from collections import deque
 
def solve(h, w, a):
    used = [[True] * (w+2) for _ in range(h+2)]
    que = deque()
    for r in range(h):
        used[r+1][1:-1] = map(lambda _:_=="#", a[r])
        for c in range(w):
            if a[r][c] == "#":
                que.append((r+1, c+1, 0))
    neighbors = [(1,0), (0,1), (-1,0), (0,-1)]
    while que:
        r, c, k = que.popleft()
        for dr, dc in neighbors:
            nr, nc = r+dr, c+dc
            if not used[nr][nc]:
                que.append((nr, nc, k+1))
                used[nr][nc] = True
    return k
 
h, w = map(int, input().split())
a = [input() for _ in range(h)]
print(solve(h, w, a))

Buggy python submission (verdict: Runtime Error):
from collections import deque

h, w = map(int,input().split())
grid = [input() for i in range(h)]

#初期の黒の位置からの距離
dist = [[-1]*w for _ in range(h)] 

#黒の位置をキューに追加（複数スタート）
black_cells = deque()
for i in range(h):
    for j in range(w):
        if grid[i][j] == '#':
            black_cells.append((h,w))
            dist[i][j] = 0

def bfs(black_cells, dist):
    d = 0
    while black_cells:
        h,w = black_cells.popleft()
        d = dist[h][w]
        for dy, dx in ((1,0),(0,1),(-1,0),(0,-1)):
            new_h = h + dh
            new_w = w + dw
            if new_h <0 or h<=new_h or new_w <0 or w <= new_w:
                continue
            if dist[new_h][new_w] == -1: # セルがしろであるのと同意
                dist[new_h][new_w] = d+1
                
                black_cells.append((new_h,new_w))
    return d

d = bfs(black_cells, dist)
print(d)

Fixed submission:
from collections import deque
 
def solve(h, w, a):
    used = [[True] * (w+2) for _ in range(h+2)]
    que = deque()
    for r in range(h):
        used[r+1][1:-1] = map(lambda _:_=="#", a[r])
        for c in range(w):
            if a[r][c] == "#":
                que.append((r+1, c+1, 0))
    neighbors = [(1,0), (0,1), (-1,0), (0,-1)]
    while que:
        r, c, k = que.popleft()
        for dr, dc in neighbors:
            nr, nc = r+dr, c+dc
            if not used[nr][nc]:
                que.append((nr, nc, k+1))
                used[nr][nc] = True
    return k
 
h, w = map(int, input().split())
a = [input() for _ in range(h)]
print(solve(h, w, a))