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):
from itertools import product
import queue
 
H, W = map(int, input().split())
road_grid = list()
for _ in range(H):
  road_grid.append(input())

print(road_grid)

# for each start point, find the furthest travellable point
# then return largest path length of all these
# ps y is down, x is right
furthest_so_far = -1
for y_init, x_init in product(range(H), range(W)):
    if road_grid[y_init][x_init] == "#": continue
    boundary = queue.Queue()
    boundary.put((y_init, x_init))
    steps_taken = [[-1]*W for _ in range(H)]
    steps_taken[y_init][x_init] = 0
    while not boundary.empty():
        y, x = boundary.get()
        neighbours = [(y+1, x), (y, x+1), (y-1, x), (y, x-1)]
        for (new_y, new_x) in neighbours:
            if (0 <= new_y < H and 0 <= new_x < W
                and steps_taken[new_y][new_x] == -1 and road_grid[new_y][new_x] == "."):
                steps_taken[new_y][new_x] = steps_taken[y][x] + 1
                boundary.put((new_y, new_x))
    furthest_so_far = max(furthest_so_far, max(max(steps_taken)))
print(furthest_so_far)

import queue
from itertools import product

test_lines = test_input.split("\n")
H, W = map(int, test_lines.pop(0).split())
road_grid = [row for row in test_lines]

# for each start point, find the furthest travellable point
# then return largest path length of all these
# ps y is down, x is right
furthest_so_far = -1
for y_init, x_init in product(range(H), range(W)):
    if road_grid[y_init][x_init] == "#": continue
    boundary = queue.Queue()
    boundary.put((y_init, x_init))
    steps_taken = [[-1]*W for _ in range(H)]
    steps_taken[y_init][x_init] = 0
    while not boundary.empty():
        y, x = boundary.get()
        neighbours = [(y+1, x), (y, x+1), (y-1, x), (y, x-1)]
        for (new_y, new_x) in neighbours:
            if (0 <= new_y < H and 0 <= new_x < W
                and steps_taken[new_y][new_x] == -1 and road_grid[new_y][new_x] == "."):
                steps_taken[new_y][new_x] = steps_taken[y][x] + 1
                boundary.put((new_y, new_x))
    furthest_so_far = max(furthest_so_far, max(max(steps_taken)))
print(furthest_so_far)


from itertools import product
import queue
 
H, W = map(int, input().split())
road_grid = list()
for _ in range(H):
  road_grid.append(input())

# for each start point, find the furthest travellable point
# then return largest path length of all these
# ps y is down, x is right
furthest_so_far = -1
for y_init, x_init in product(range(H), range(W)):
    if road_grid[y_init][x_init] == "#": continue
    boundary = queue.Queue()
    boundary.put((y_init, x_init))
    steps_taken = [[-1]*W for _ in range(H)]
    steps_taken[y_init][x_init] = 0
    while not boundary.empty():
        y, x = boundary.get()
        neighbours = [(y+1, x), (y, x+1), (y-1, x), (y, x-1)]
        for (new_y, new_x) in neighbours:
            if (0 <= new_y < H and 0 <= new_x < W
                and steps_taken[new_y][new_x] == -1 and road_grid[new_y][new_x] == "."):
                steps_taken[new_y][new_x] = steps_taken[y][x] + 1
                boundary.put((new_y, new_x))
    furthest_here = max(max(row) for row in steps_taken)
    furthest_so_far = max(furthest_so_far, furthest_here)
print(furthest_so_far)

Verdicts: Wrong Answer

Runtime Error

Accepted