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):
work = [[[] for j in range(w)] for i in range(h)]
result = 0
for i in range(h):
    for j in range(w):
        append = work[i][j].append
        if i<h-1 and s[i+1][j] == ".":
            append([i+1,j])
        if i>0 and s[i-1][j] == ".":
            append([i-1,j])
        if j<w-1 and s[i][j+1] == ".":
            append([i,j+1])
        if j>0 and s[i][j-1] == ".":
            append([i,j-1])
for i in range(h):
    for j in range(w):
        if s[i][j] != ".":
            continue
        q = deque([[i, j]])
        temp = [[i, j]]
        popleft = q.popleft
        append = q.append
        d = -1
        count = 1
        counter = 0
        for k in range(h*w):
            x, y = popleft()
            count -= 1
            for el in work[x][y]:
                if el not in temp:
                    counter += 1
                    append(el)
                    temp.append(el)
            if count == 0:
                d += 1
                count = counter
                counter = 0
            if len(q) == 0:
                break
        result = result if result>d else d
print(result)

from collections import deque
h, w = map(int, input().split())
s = [list(input()) for i in range(h)]
work = [[[] for j in range(w)] for i in range(h)]
result = 0
for i in range(h):
    for j in range(w):
        if i<h-1 and s[i+1][j] == ".":
            work[i][j].append([i+1,j])
        if i>0 and s[i-1][j] == ".":
            work[i][j].append([i-1,j])
        if j<w-1 and s[i][j+1] == ".":
            work[i][j].append([i,j+1])
        if j>0 and s[i][j-1] == ".":
            work[i][j].append([i,j-1])
for i in range(h):
    for j in range(w):
        if s[i][j] != ".":
            continue
        stack = deque([[i, j]])
        temp = [[i, j]]
        distance = 0
        for k in range(h*w):
            x, y = stack.pop()
            flag = False
            for el in work[x][y]:
                if el not in temp:
                    flag = True
                    stack.append(el)
                    temp.append(el)
            if flag:
                distance += 1
            else:
                result = result if result > distance else distance
                distance = -1
            if len(stack) == 0:
                break
print(result)

from collections import deque
h, w = map(int, input().split())
s = [list(input()) for i in range(h)]
work = [[[] for j in range(w)] for i in range(h)]
result = 0
for i in range(h):
    for j in range(w):
        append = work[i][j].append
        if i<h-1 and s[i+1][j] == ".":
            append([i+1,j])
        if i>0 and s[i-1][j] == ".":
            append([i-1,j])
        if j<w-1 and s[i][j+1] == ".":
            append([i,j+1])
        if j>0 and s[i][j-1] == ".":
            append([i,j-1])
q = deque()
popleft = q.popleft
append = q.append
for i in range(h):
    for j in range(w):
        if s[i][j] != ".":
            continue
        q.clear()
        q.append([i, j])
        temp = [[i, j]]
        tempappend = temp.append
        d = -1
        count = 1
        counter = 0
        for k in range(w*h):
            x, y = popleft()
            count -= 1
            for el in work[x][y]:
                if el not in temp:
                    counter += 1
                    append(el)
                    tempappend(el)
            if count == 0:
                d += 1
                count = counter
                counter = 0
            if len(q) == 0:
                break
        result = result if result>d else d
    if result == h+w-2:
        break
print(result)

Verdicts: Runtime Error

Wrong Answer

Accepted