Problem: Takahashi has a maze, which is a grid of H \times W squares with H horizontal rows and W vertical columns.  The square at the i-th row from the top and the j-th column is a "wall" square if S_{ij} is `#`, and a "road" square if S_{ij} is `.`.  From a road square, you can move to a horizontally or vertically adjacent road square.  You cannot move out of the maze, move to a wall square, or move diagonally.  Takahashi will choose a starting square and a goal square, which can be any road squares, and give the maze to Aoki.  Aoki will then travel from the starting square to the goal square, in the minimum number of moves required.  In this situation, find the maximum possible number of moves Aoki has to make.

Submission attempts (in order):
import queue

h, w = map(int, input().split())
mat = ["#"*(w+2)]
for y in range(h):
  mat.append("#"+input()+"#")
mat.append("#"*(w+2))

d_mat1 = [[-1]*(w+2) for i in range(h+2)]
d_mat2 = [[-1]*(w+2) for i in range(h+2)]
q = queue.Queue()
for x in range(1, w+1):
  for y in range(1, h+1):
    if mat[x][y] == ".":
      start = (x, y)
      d_mat1[x][y] = 0
q.put(start)

while not q.empty():
  x, y = q.get()
  d = d_mat1[x][y]
  if mat[x-1][y] == "." and d_mat1[x-1][y] == -1:
    d_mat1[x-1][y] = d+1
    q.put((x-1, y))
    
  if mat[x+1][y] == "." and d_mat1[x+1][y] == -1:
    d_mat1[x+1][y] = d+1
    q.put((x+1, y))
    
  if mat[x][y-1] == "." and d_mat1[x][y-1] == -1:
    d_mat1[x][y-1] = d+1
    q.put((x, y-1))
    
  if mat[x][y+1] == "." and d_mat1[x][y+1] == -1:
    d_mat1[x][y+1] = d+1
    q.put((x, y+1))

end = (x,y)
d_mat2[x][y] = 0
q.put(end)
while not q.empty():
  x, y = q.get()
  d = d_mat1[x][y]
  if mat[x-1][y] == "." and d_mat2[x-1][y] == -1:
    d_mat2[x-1][y] = d+1
    q.put((x-1, y))
    
  if mat[x+1][y] == "." and d_mat2[x+1][y] == -1:
    d_mat2[x+1][y] = d+1
    q.put((x+1, y))
    
  if mat[x][y-1] == "." and d_mat2[x][y-1] == -1:
    d_mat2[x][y-1] = d+1
    q.put((x, y-1))
    
  if mat[x][y+1] == "." and d_mat2[x][y+1] == -1:
    d_mat2[x][y+1] = d+1
    q.put((x, y+1))

print(d_mat2[x][y])

import queue

def bfs(start):
  d_mat = [[-1]*(w+2) for i in range(h+2)]
  q.put(start)
  d_mat[start[0]][start[1]] = 0
  while not q.empty():
    x, y = q.get()
    d = d_mat[x][y]
    if mat[x-1][y] == "." and d_mat[x-1][y] == -1:
      d_mat[x-1][y] = d+1
      q.put((x-1, y))
    
    if mat[x+1][y] == "." and d_mat[x+1][y] == -1:
      d_mat[x+1][y] = d+1
      q.put((x+1, y))
    
    if mat[x][y-1] == "." and d_mat[x][y-1] == -1:
      d_mat[x][y-1] = d+1
      q.put((x, y-1))
    
    if mat[x][y+1] == "." and d_mat[x][y+1] == -1:
      d_mat[x][y+1] = d+1
      q.put((x, y+1))
  return d_mat[x][y]
h, w = map(int, input().split())
mat = ["#"*(w+2)]
for y in range(h):
  mat.append("#"+input()+"#")
mat.append("#"*(w+2))

d_mat1 = [[-1]*(w+2) for i in range(h+2)]
d_mat2 = [[-1]*(w+2) for i in range(h+2)]
q = queue.Queue()
start_list = []
flag = True
for x in range(1, w+1):
  if flag:
    for y in range(1, h+1):
      if mat[y][x] == ".":
        start = (y, x)
        start_list.append(start)
        
q.put(start)

while not q.empty():
  x, y = q.get()
  d = d_mat1[x][y]
  if mat[x-1][y] == "." and d_mat1[x-1][y] == -1:
    d_mat1[x-1][y] = d+1
    q.put((x-1, y))
    
  if mat[x+1][y] == "." and d_mat1[x+1][y] == -1:
    d_mat1[x+1][y] = d+1
    q.put((x+1, y))
    
  if mat[x][y-1] == "." and d_mat1[x][y-1] == -1:
    d_mat1[x][y-1] = d+1
    q.put((x, y-1))
    
  if mat[x][y+1] == "." and d_mat1[x][y+1] == -1:
    d_mat1[x][y+1] = d+1
    q.put((x, y+1))



print(max(list(map(bfs, start_list))))


Verdicts: Runtime Error

Accepted