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):
#!/usr/bin/env python
# -*- coding: utf-8 -*-
import sys
import os
f = open('input.txt', 'r')
sys.stdin = f
input = sys.stdin.readline



# D maze master

h, w = map(int, input().split())

graph = {i: {} for i in range(1, h * w + 1)}

data = []
data.append([False for i in range(w + 2)])
for i in range(h):
    s = list(input())
    sub = [False]
    for i in range(w):
        if s[i] is "#":
            sub.append(False)
        else:
            sub.append(True)
    sub.append(False)
    data.append(sub)
data.append([False for i in range(2 + w)])

for i in range(1, h + 1):
    for j in range(1, w + 1):
        # now data[i][j]
        if data[i][j]:
            now = w * (i - 1) + j
            if data[i - 1][j]:
                node = w * (i - 1) + j
                graph[now][node]=1
                graph[node][now]=1
            if data[i][j + 1]:
                node = w* (i - 1) + j + 1
                graph[now][node]=1
                graph[node][now]=1
            if data[i][j - 1]:
                node = w * (i - 1) + j - 1
                graph[node][now]=1
                graph[now][node]=1
            if data[i + 1][j]:
                node = w * i + j
                graph[node][now]=1
                graph[now][node]=1



import heapq
def dijkstra(s,graph,n):
    
    dist=[10**20 for i in range(n+1)]
    dist[s]=0
    pq=[]
    heapq.heapify(pq)
    
    heapq.heappush(pq,(0,s))
    while pq:
        x=heapq.heappop(pq)
        node,mini_dis=x[1],x[0]
        for point in graph[node].keys():
            w=graph[node][point]
            
            newlen=dist[node]+w
            if newlen<dist[point]:
                heapq.heappush(pq,(newlen,point))
                dist[point]=newlen
    mea=0
    for i in range(1,n+1):
        if 0<dist[i]<10**20:
            mea=max(dist[i],mea)
    return mea

###############################################################
ans=0
for i in range(1,h*w+1):
    if graph[i]:
        ans=max(ans,dijkstra(i,graph,h*w))
print(ans)


h,w=map(int,input().split())
lists=[]
from collections import deque
d=deque()
e=deque()
field=[]
distance=0
field.append([True for i in range(w+2)])
for i in range(h):
    sub=[True]
    s=input()
    for j in range(w):
        if s[j]=="#":
            sub.append(True)
        else:
            sub.append(False)
            lists.append((i+1,j+1))
    sub.append(True)
    field.append(sub)
field.append([True for i  in range(w+2)]  )           

fieldss=field.copy()
maxi=0
for i in range(len(lists)):
        distance=0
        fields=fieldss
        sy,sx=lists[i][0],lists[i][1]
        d=deque()
        e=deque()
        d.append((sy,sx))
        fields[sy][sx]=True
        while d:
            for some in d:
                some=d.popleft()
                index1,index2=some[0],some[1]

                if fields[index1-1][index2]==False:
                    fields[index1-1][index2]=True
                    e.append((index1-1,index2))
                    
                if fields[index1+1][index2]==False:
                    fields[index1+1][index2]=True
                    e.append((index1+1,index2))
                    
                if fields[index1][index2+1]==False:
                    fields[index1][index2+1]=True
                    e.append((index1,index2+1))
                    
                if  fields[index1][index2-1]==False:
                    fields[index1][index2-1]=True
                    e.append((index1,index2-1))
        distance+=1
        d=e
        e=deque()
        maxi=max(maxi,distance)
print(maxi)       
        


from collections import deque
H, W = map(int, input().split())
dif = [(-1, 0), (1, 0), (0, 1), (0, -1)]
Map = [list(input()) for i in range(H)]
visited = [[False for i in range(W)] for j in range(H)]
deq = deque()


def bfs(node):
    nx, ny = node
    deq.append(node)

    visited[nx][ny] = True
    distance = [[float("inf") for i in range(W)] for j in range(H)]
    distance[nx][ny] = 0
    while deq:
        x, y = deq.popleft()
        for dx, dy in dif:
            if 0 <= x+dx < H and 0 <= y+dy < W and Map[x+dx][y+dy] == "." and visited[x+dx][y+dy] == False:
                visited[x+dx][y+dy] = True
                distance[x+dx][y+dy] = distance[x][y]+1
                deq.append((x+dx, y+dy))
    return distance


def s_bfs(node):
    nx, ny = node
    deq.append(node)
    Visited = [[False for i in range(W)] for j in range(H)]
    Visited[nx][ny] = True
    distance = [[float("inf") for i in range(W)] for j in range(H)]
    distance[nx][ny] = 0
    while deq:
        x, y = deq.popleft()
        for dx, dy in dif:
            if 0 <= x+dx < H and 0 <= y+dy < W and Map[x+dx][y+dy] == "." and Visited[x+dx][y+dy] == False:
                Visited[x+dx][y+dy] = True
                distance[x+dx][y+dy] = distance[x][y]+1
                deq.append((x+dx, y+dy))
    return distance


ans = 1
roads = deque()
for i in range(H):
    for j in range(W):
        if Map[i][j] == ".":
            roads.append((i, j))

while roads:
    x, y = roads.popleft()
    if visited[x][y] == False:
        B = s_bfs((x, y))
        s = -1
        choice=[]
        most_far_distance = 0
        for i in range(H):
            for j in range(W):
                if B[i][j] != float("inf") and B[i][j] > most_far_distance:
                    most_far_distance = B[i][j]
        for i in range(H):
            for j in range(W):
                if B[i][j] == most_far_distance:
                    choice.append((i, j))

        for s in choice:
            C = s_bfs(s)
            for i in range(H):
                for j in range(W):
                    if C[i][j] != float("inf"):
                        ans = max(ans, C[i][j])
print(ans)

Verdicts: Runtime Error

Runtime Error

Accepted