Buggy python submission (verdict: Time Limit Exceeded):
def resolve():
    H, W = map(int, input().split())
    s = [input() for _ in range(H)]

    def bfs(row, col, R, C, s):
        from collections import deque
        q = deque()
        q.append([(row, col), 0])
        seen = set()
        ans = 0
        while q:
            (r, c), dist = q.popleft()
            ans = max(dist, ans)
            seen.add((r, c))
            for (next_r, next_c) in ((r+1, c), (r-1, c), (r, c+1), (r, c-1)):
                if (next_r, next_c) not in seen and \
                    0 <= next_r < R and 0 <= next_c < C and \
                   s[next_r][next_c] == '.':
                    q.append([(next_r, next_c), dist + 1])
        return ans

    max_v = 0
    for h in range(H):
        for w in range(W):
            if s[h][w] == '.':
                max_v = max(max_v, bfs(h, w, H, W, s))
    print(max_v)
resolve()

Fixed submission:
def resolve():
    H, W = map(int, input().split())
    s = [input() for _ in range(H)]
    from collections import defaultdict
    cnt = defaultdict(int)
    for item in s:
        for c in item:
            cnt[c] += 1
    if '#' not in cnt:
        print(H+W-2)
        quit()

    def bfs(row, col, R, C, s):
        from collections import deque
        q = deque()
        q.append([(row, col), 0])
        seen = set()
        ans = 0
        while q:
            (r, c), dist = q.popleft()
            ans = max(dist, ans)
            seen.add((r, c))
            for (next_r, next_c) in ((r+1, c), (r-1, c), (r, c+1), (r, c-1)):
                if (next_r, next_c) not in seen and \
                    0 <= next_r < R and 0 <= next_c < C and \
                   s[next_r][next_c] == '.':
                    q.append([(next_r, next_c), dist + 1])
        return ans

    max_v = 0
    for h in range(H):
        for w in range(W):
            if s[h][w] == '.':
                max_v = max(max_v, bfs(h, w, H, W, s))
    print(max_v)
resolve()

Buggy python submission (verdict: Time Limit Exceeded):
def resolve():
    H, W = map(int, input().split())
    s = [input() for _ in range(H)]

    def bfs(row, col, R, C, s):
        from collections import deque
        q = deque()
        q.append([(row, col), 0])
        seen = set()
        ans = 0
        while q:
            (r, c), dist = q.popleft()
            ans = max(dist, ans)
            seen.add((r, c))
            for (next_r, next_c) in ((r+1, c), (r-1, c), (r, c+1), (r, c-1)):
                if 0 <= next_r < R and 0 <= next_c < C and \
                   (next_r, next_c) not in seen and \
                   s[next_r][next_c] == '.':
                    q.append([(next_r, next_c), dist + 1])
        return ans

    max_v = 0
    for h in range(H):
        for w in range(W):
            if s[h][w] == '.':
                max_v = max(max_v, bfs(h, w, H, W, s))
    print(max_v)
resolve()

Fixed submission:
def resolve():
    H, W = map(int, input().split())
    s = [input() for _ in range(H)]
    from collections import defaultdict
    cnt = defaultdict(int)
    for item in s:
        for c in item:
            cnt[c] += 1
    if '#' not in cnt:
        print(H+W-2)
        quit()

    def bfs(row, col, R, C, s):
        from collections import deque
        q = deque()
        q.append([(row, col), 0])
        seen = set()
        ans = 0
        while q:
            (r, c), dist = q.popleft()
            ans = max(dist, ans)
            seen.add((r, c))
            for (next_r, next_c) in ((r+1, c), (r-1, c), (r, c+1), (r, c-1)):
                if (next_r, next_c) not in seen and \
                    0 <= next_r < R and 0 <= next_c < C and \
                   s[next_r][next_c] == '.':
                    q.append([(next_r, next_c), dist + 1])
        return ans

    max_v = 0
    for h in range(H):
        for w in range(W):
            if s[h][w] == '.':
                max_v = max(max_v, bfs(h, w, H, W, s))
    print(max_v)
resolve()