Buggy python submission (verdict: Runtime Error):
#幅優先探索


from queue import Queue

ans = 0
h,w = map(int, input().split())
maps = [input() for _ in range(h)]
for i in range(1):
    for j in range(1,2):
        if maps[i][j] == '.':
            visited = [[-1]*w for _ in range(h)]
            q = Queue()
            q.put([i,j])
            visited[i][j] = 0
            while not q.empty():
                y,x = q.get()
                for k,l in [[1,0],[-1,0],[0,1],[0,-1]]:
                    if 0 <= y+k < h and 0<= x+l < w:
                        new_y,new_x = y+k,x+l
                    else: continue
                    if visited[new_y][new_x]== -1 and maps[new_y][new_x]==".":
                        visited[new_y][new_x]=visited[y][x]+1
                        q.put([new_y,new_x])
   
          
        
            for ii in range(h):
                for jj in range(w):
                    ans = max(ans,visited[ii][jj])


print(ans)




Fixed submission:
#幅優先探索


from queue import Queue

ans = 0
h,w = map(int, input().split())
maps = [input() for _ in range(h)]
for i in range(h):
    for j in range(w):
        if maps[i][j] == '.':
            visited = [[-1]*w for _ in range(h)]
            q = Queue()
            q.put([i,j])
            visited[i][j] = 0
            while not q.empty():
                y,x = q.get()
                for k,l in [[1,0],[-1,0],[0,1],[0,-1]]:
                    if 0 <= y+k < h and 0<= x+l < w:
                        new_y,new_x = y+k,x+l
                    else: continue
                    if visited[new_y][new_x]== -1 and maps[new_y][new_x]==".":
                        visited[new_y][new_x]=visited[y][x]+1
                        q.put([new_y,new_x])
   
          
        
            for ii in range(h):
                for jj in range(w):
                    ans = max(ans,visited[ii][jj])


print(ans)

Buggy python submission (verdict: Runtime Error):
#幅優先探索


from queue import Queue

ans = 0
h,w = map(int, input().split())
maps = [input() for _ in range(h)]
for i in range(h):
    for j in range(w):
        if maps[i][j] == '.':
            visited = [[-1]*w for _ in range(h)]
            q = Queue()
            q.put([i,j])
            visited[i][j] = 0
            while not q.empty():
                y,x = q.get()
                for k,l in [[1,0],[-1,0],[0,1],[0,-1]]:
                    if 0 <= y+k < h and 0<= x+l < w:
                        new_y,new_x = y+k,x+l
                    else: continue
                    if visited[new_y][new_x]== -1 and maps[new_y][new_x]==".":
                        visited[new_y][new_x]=visited[y][x]+1
                        q.put([new_y,new_x])
                    elif visited[new_y][new_x] > visited[y][x]+1:
                        visited[new_y][new_x]=visited[y][x]+1
                        q.put([new_y,new_x])
          
         
    for ii in range(h):
        for jj in range(w):
            ans = max(ans,visited[ii][jj])
        
print(ans)





Fixed submission:
#幅優先探索


from queue import Queue

ans = 0
h,w = map(int, input().split())
maps = [input() for _ in range(h)]
for i in range(h):
    for j in range(w):
        if maps[i][j] == '.':
            visited = [[-1]*w for _ in range(h)]
            q = Queue()
            q.put([i,j])
            visited[i][j] = 0
            while not q.empty():
                y,x = q.get()
                for k,l in [[1,0],[-1,0],[0,1],[0,-1]]:
                    if 0 <= y+k < h and 0<= x+l < w:
                        new_y,new_x = y+k,x+l
                    else: continue
                    if visited[new_y][new_x]== -1 and maps[new_y][new_x]==".":
                        visited[new_y][new_x]=visited[y][x]+1
                        q.put([new_y,new_x])
   
          
        
            for ii in range(h):
                for jj in range(w):
                    ans = max(ans,visited[ii][jj])


print(ans)