Buggy python submission (verdict: Time Limit Exceeded):
H,W = map(int,input().split())
maze = [list(input()) for j in range(H)]

def bfs(h,w):
  checked = [[False for i in range(W)] for j in range(H)]
  q = [(h,w,0)] if maze[h][w]=="." else []
  g_n = 0
  while q:
    h_,w_,n = q.pop(0)
    checked[h_][w_] = True
    g_n = max(n, g_n)
    for i,j in zip([0,0,-1,1],[-1,1,0,0]):
      if (0<=h_+i<H) and (0<=w_+j<W):
        if checked[h_+i][w_+j]: continue
        if maze[h_+i][w_+j]==".": q.append((h_+i,w_+j,n+1))
  return g_n

n = 0
for i in range(H):
  for j in range(W):
    n = max(n, bfs(i,j))
print(n)

Fixed submission:
H,W = map(int,input().split())
maze = [list(input()) for j in range(H)]

def bfs(h,w):
  checked = [[False for i in range(W)] for j in range(H)]
  q = [(h,w,0)] if maze[h][w]=="." else []
  checked[h][w] = True
  g_n = 0
  while q:
    h_,w_,n = q.pop(0)
    g_n = max(n, g_n)
    for i,j in zip([0,0,-1,1],[-1,1,0,0]):
      if (0<=h_+i<H) and (0<=w_+j<W):
        if checked[h_+i][w_+j]: continue
        checked[h_+i][w_+j] = True
        if maze[h_+i][w_+j]==".": q.append((h_+i,w_+j,n+1))
  return g_n

n = 0
for i in range(H):
  for j in range(W):
    n = max(n, bfs(i,j))
print(n)

Buggy python submission (verdict: Runtime Error):
H,W = map(int,input().split())
maze = [list(input()) for j in range(H)]

def bfs(h,w):
  checked = [[False for i in range(W)] for j in range(H)]
  q = [(h,w,0)] if maze[h][w]=="." else []
  g_n = 0
  while q:
    h_,w_,n = q.pop(0)
    g_n = max(n, g_n)
    for i,j in zip([0,0,-1,1],[-1,1,0,0]):
      if (0<=h_+i<H) and (0<=w_+j<W):
        if checked[h_+i][w_+j]: continue
        checked[h_+i][w_+j] = True
        if maze[pt[0]][pt[1]]==".": q.append((h_+i,w_+j,n+1))
  return g_n

n = 0
for i in range(H):
  for j in range(W):
    n = max(n, bfs(i,j))
print(n)

Fixed submission:
H,W = map(int,input().split())
maze = [list(input()) for j in range(H)]

def bfs(h,w):
  checked = [[False for i in range(W)] for j in range(H)]
  q = [(h,w,0)] if maze[h][w]=="." else []
  checked[h][w] = True
  g_n = 0
  while q:
    h_,w_,n = q.pop(0)
    g_n = max(n, g_n)
    for i,j in zip([0,0,-1,1],[-1,1,0,0]):
      if (0<=h_+i<H) and (0<=w_+j<W):
        if checked[h_+i][w_+j]: continue
        checked[h_+i][w_+j] = True
        if maze[h_+i][w_+j]==".": q.append((h_+i,w_+j,n+1))
  return g_n

n = 0
for i in range(H):
  for j in range(W):
    n = max(n, bfs(i,j))
print(n)

Buggy python submission (verdict: Time Limit Exceeded):
H,W = map(int,input().split())
maze = [list(input()) for j in range(H)]

def bfs(h,w):
  checked = [(h,w)]
  q = [(h,w,0)] if maze[h][w]=="." else []
  g_n = 0
  while q:
    h_,w_,n = q.pop(0)
    g_n = max(n, g_n)
    for i,j in zip([0,0,-1,1],[-1,1,0,0]):
      if (0<=h_+i<H) and (0<=w_+j<W):
        pt = (h_+i,w_+j)
        if pt in checked: continue
        checked.append(pt)
        q = q + [pt+(n+1,)] if maze[pt[0]][pt[1]]=="." else q
  return g_n

n = 0
for i in range(H):
  for j in range(W):
    n = max(n, bfs(i,j))
print(n)

Fixed submission:
H,W = map(int,input().split())
maze = [list(input()) for j in range(H)]

def bfs(h,w):
  checked = [[False for i in range(W)] for j in range(H)]
  q = [(h,w,0)] if maze[h][w]=="." else []
  checked[h][w] = True
  g_n = 0
  while q:
    h_,w_,n = q.pop(0)
    g_n = max(n, g_n)
    for i,j in zip([0,0,-1,1],[-1,1,0,0]):
      if (0<=h_+i<H) and (0<=w_+j<W):
        if checked[h_+i][w_+j]: continue
        checked[h_+i][w_+j] = True
        if maze[h_+i][w_+j]==".": q.append((h_+i,w_+j,n+1))
  return g_n

n = 0
for i in range(H):
  for j in range(W):
    n = max(n, bfs(i,j))
print(n)