Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
def bfs(maze, visited, l,n_w):
    queue = deque(l)
    for i in queue:
        visited[i[0]][i[1]] = 0
    while queue:
        y, x = queue.popleft()
        if n_w==0:
            s=set()
            tmp=0
            for i in visited:
                if max(i)>tmp:tmp=max(i)
            return(tmp)
        for j, k in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y+j, x+k
            if maze[new_y][new_x] == "." and visited[new_y][new_x] == -1:
                visited[new_y][new_x] = visited[y][x] + 1
                queue.append([new_y, new_x])
                n_w-=1

R, C = map(int, input().split())


maze = ["$"*(C+2)]+["$"+input()+"$" for i in range(R)]+["$"*(C+2)]
visited = [[-2]*(C+2)]+[[-2]+[-1]*(C)+[-2] for j in range(R)]+[[-2]*(C+2)]
l_b=[]
for i in range(R+2):
    for j in range(C+2):
        if maze[i][j]=="#":
            l_b.append([i,j])
n_w=R*C-len(l_b)

print(bfs(maze, visited,l_b,n_w))

Fixed submission:
from collections import deque
def bfs(maze, visited, l,n_w):
    queue = deque(l)
    for i in queue:
        visited[i[0]][i[1]] = 0
    while queue:
        y, x = queue.popleft()
        if n_w==0:
            s=set()
            tmp=0
            for i in visited:
                if max(i)>tmp:tmp=max(i)
            return(tmp)
        for j, k in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y+j, x+k
            if maze[new_y][new_x] == "." and visited[new_y][new_x] == -1:
                visited[new_y][new_x] = visited[y][x] + 1
                queue.append([new_y, new_x])
                n_w-=1

R, C = map(int, input().split())


maze = ["$"*(C+2)]+["$"+input()+"$" for i in range(R)]+["$"*(C+2)]
visited = [[-2]*(C+2)]+[[-2]+[-1]*(C)+[-2] for j in range(R)]+[[-2]*(C+2)]
l_b=[]
for i in range(R+2):
    for j in range(C+2):
        if maze[i][j]=="#":
            l_b.append([i,j])
n_w=R*C-len(l_b)

print(bfs(maze, visited,l_b,n_w))

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
def bfs(maze, visited, l,n_w):
    queue = deque(l)
    for i in queue:
        visited[i[0]][i[1]] = 0
    while queue:
        y, x = queue.popleft()
        if n_w==0:
            s=set()
            for i in visited:
                s=s|set(i)
            return(max(s))
        for j, k in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y+j, x+k
            if maze[new_y][new_x] == "." and visited[new_y][new_x] == -1:
                visited[new_y][new_x] = visited[y][x] + 1
                queue.append([new_y, new_x])
                n_w-=1

R, C = map(int, input().split())


maze = ["$"*(C+2)]+["$"+input()+"$" for i in range(R)]+["$"*(C+2)]
visited = [[-2]*(C+2)]+[[-2]+[-1]*(C)+[-2] for j in range(R)]+[[-2]*(C+2)]
l_b=[]
for i in range(R+2):
    for j in range(C+2):
        if maze[i][j]=="#":
            l_b.append([i,j])
n_w=R*C-len(l_b)

print(bfs(maze, visited,l_b,n_w))

Fixed submission:
from collections import deque
def bfs(maze, visited, l,n_w):
    queue = deque(l)
    for i in queue:
        visited[i[0]][i[1]] = 0
    while queue:
        y, x = queue.popleft()
        if n_w==0:
            s=set()
            tmp=0
            for i in visited:
                if max(i)>tmp:tmp=max(i)
            return(tmp)
        for j, k in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y+j, x+k
            if maze[new_y][new_x] == "." and visited[new_y][new_x] == -1:
                visited[new_y][new_x] = visited[y][x] + 1
                queue.append([new_y, new_x])
                n_w-=1

R, C = map(int, input().split())


maze = ["$"*(C+2)]+["$"+input()+"$" for i in range(R)]+["$"*(C+2)]
visited = [[-2]*(C+2)]+[[-2]+[-1]*(C)+[-2] for j in range(R)]+[[-2]*(C+2)]
l_b=[]
for i in range(R+2):
    for j in range(C+2):
        if maze[i][j]=="#":
            l_b.append([i,j])
n_w=R*C-len(l_b)

print(bfs(maze, visited,l_b,n_w))

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
def bfs(maze, visited, l):
    queue = deque(l)
    for i in queue:
        visited[i[0]][i[1]] = 0
    while queue:
        y, x = queue.popleft()
        if all ([-1 not in i for i in visited]):
            s=set()
            for i in visited:
                s=s|set(i)
            return(max(s))
        for j, k in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y+j, x+k
            if maze[new_y][new_x] == "." and visited[new_y][new_x] == -1:
                visited[new_y][new_x] = visited[y][x] + 1
                queue.append([new_y, new_x])

R, C = map(int, input().split())


maze = ["$"*(C+2)]+["$"+input()+"$" for i in range(R)]+["$"*(C+2)]
visited = [[-2]*(C+2)]+[[-2]+[-1]*(C)+[-2] for j in range(R)]+[[-2]*(C+2)]
l_b=[]
for i in range(R+2):
    for j in range(C+2):
        if maze[i][j]=="#":
            l_b.append([i,j])

print(bfs(maze, visited,l_b))

Fixed submission:
from collections import deque
def bfs(maze, visited, l,n_w):
    queue = deque(l)
    for i in queue:
        visited[i[0]][i[1]] = 0
    while queue:
        y, x = queue.popleft()
        if n_w==0:
            s=set()
            tmp=0
            for i in visited:
                if max(i)>tmp:tmp=max(i)
            return(tmp)
        for j, k in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y+j, x+k
            if maze[new_y][new_x] == "." and visited[new_y][new_x] == -1:
                visited[new_y][new_x] = visited[y][x] + 1
                queue.append([new_y, new_x])
                n_w-=1

R, C = map(int, input().split())


maze = ["$"*(C+2)]+["$"+input()+"$" for i in range(R)]+["$"*(C+2)]
visited = [[-2]*(C+2)]+[[-2]+[-1]*(C)+[-2] for j in range(R)]+[[-2]*(C+2)]
l_b=[]
for i in range(R+2):
    for j in range(C+2):
        if maze[i][j]=="#":
            l_b.append([i,j])
n_w=R*C-len(l_b)

print(bfs(maze, visited,l_b,n_w))

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
def bfs(maze, visited, l):
    queue = deque(l)
    for i in queue:
        visited[i[0]][i[1]] = 0
    while queue:
        y, x = queue.popleft()
        if all ([-1 not in i for i in visited]):
            s=set()
            for i in visited:
                s.add(max(i))
            return(max(s))
        for j, k in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y+j, x+k
            if maze[new_y][new_x] == "." and visited[new_y][new_x] == -1:
                visited[new_y][new_x] = visited[y][x] + 1
                queue.append([new_y, new_x])

R, C = map(int, input().split())


maze = ["$"*(C+2)]+["$"+input()+"$" for i in range(R)]+["$"*(C+2)]
visited = [[-2]*(C+2)]+[[-2]+[-1]*(C)+[-2] for j in range(R)]+[[-2]*(C+2)]
l_b=[]
for i in range(R+2):
    for j in range(C+2):
        if maze[i][j]=="#":
            l_b.append([i,j])

print(bfs(maze, visited,l_b))

Fixed submission:
from collections import deque
def bfs(maze, visited, l,n_w):
    queue = deque(l)
    for i in queue:
        visited[i[0]][i[1]] = 0
    while queue:
        y, x = queue.popleft()
        if n_w==0:
            s=set()
            tmp=0
            for i in visited:
                if max(i)>tmp:tmp=max(i)
            return(tmp)
        for j, k in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y+j, x+k
            if maze[new_y][new_x] == "." and visited[new_y][new_x] == -1:
                visited[new_y][new_x] = visited[y][x] + 1
                queue.append([new_y, new_x])
                n_w-=1

R, C = map(int, input().split())


maze = ["$"*(C+2)]+["$"+input()+"$" for i in range(R)]+["$"*(C+2)]
visited = [[-2]*(C+2)]+[[-2]+[-1]*(C)+[-2] for j in range(R)]+[[-2]*(C+2)]
l_b=[]
for i in range(R+2):
    for j in range(C+2):
        if maze[i][j]=="#":
            l_b.append([i,j])
n_w=R*C-len(l_b)

print(bfs(maze, visited,l_b,n_w))