Buggy python submission (verdict: Time Limit Exceeded):
#!/usr/bin/env python3
import sys
from pprint import pprint

def solve(H: int, W: int, A: "List[str]"):
    used = [[False] * W for _ in range(H)]
    dx = [1, 0, -1, 0]
    dy = [0, -1, 0, 1]
    s = set()
    for i in range(H):
        for j in range(W):
            if A[i][j] == '#':
                s.add((i, j))
                used[i][j] = True

    ret = 0
    while True:
        nex = set()
        for x, y in s:
            for j in range(4):
                tx = x + dx[j]
                ty = y + dy[j]
                if 0 <= tx < H and 0 <= ty < W and not used[tx][ty]:
                    nex.add((tx, ty))
                    used[tx][ty] = True
        if len(nex) < 1:
            break
        s = nex
        ret += 1

    print(ret)
    return


def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    H = int(next(tokens))  # type: int
    W = int(next(tokens))  # type: int
    A = [next(tokens) for _ in range(H) ]  # type: "List[str]"
    solve(H, W, A)

if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys
from pprint import pprint

def solve(H: int, W: int, A: "List[str]"):
    used = [[False] * W for _ in range(H)]
    dx = [1, 0, -1, 0]
    dy = [0, -1, 0, 1]
    s = set()
    for i in range(H):
        for j in range(W):
            if A[i][j] == '#':
                s.add((i, j))
                used[i][j] = True

    ret = 0
    while True:
        nex = set()
        for x, y in s:
            for j in range(4):
                tx = x + dx[j]
                ty = y + dy[j]
                if 0 <= tx < H and 0 <= ty < W and not used[tx][ty]:
                    nex.add((tx, ty))
                    used[tx][ty] = True
        if len(nex) < 1:
            break
        s = nex
        ret += 1

    print(ret)
    return


def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    H = int(next(tokens))  # type: int
    W = int(next(tokens))  # type: int
    A = [next(tokens) for _ in range(H) ]  # type: "List[str]"
    solve(H, W, A)

if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
#!/usr/bin/env python3
import sys
from pprint import pprint

def solve(H: int, W: int, A: "List[str]"):
    count = [[H + W] * (W + 2) for _ in range(H + 2)]
    for i in range(H):
        for j in range(W):
            if A[i][j] == '#':
                count[i + 1][j + 1] = 0
            else:
                count[i + 1][j + 1] = min(count[i][j + 1], count[i + 1][j]) + 1

    #pprint(count)
    ret = 0
    for i in range(H, 0, -1):
        for j in range(W, 0, -1):
            count[i][j] = min(count[i][j], min(count[i][j + 1], count[i + 1][j]) + 1)
            ret = max(ret, count[i][j])
    #pprint(count)
    print(ret)
    return


def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    H = int(next(tokens))  # type: int
    W = int(next(tokens))  # type: int
    A = [next(tokens) for _ in range(H) ]  # type: "List[str]"
    solve(H, W, A)

if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys
from pprint import pprint

def solve(H: int, W: int, A: "List[str]"):
    used = [[False] * W for _ in range(H)]
    dx = [1, 0, -1, 0]
    dy = [0, -1, 0, 1]
    s = set()
    for i in range(H):
        for j in range(W):
            if A[i][j] == '#':
                s.add((i, j))
                used[i][j] = True

    ret = 0
    while True:
        nex = set()
        for x, y in s:
            for j in range(4):
                tx = x + dx[j]
                ty = y + dy[j]
                if 0 <= tx < H and 0 <= ty < W and not used[tx][ty]:
                    nex.add((tx, ty))
                    used[tx][ty] = True
        if len(nex) < 1:
            break
        s = nex
        ret += 1

    print(ret)
    return


def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    H = int(next(tokens))  # type: int
    W = int(next(tokens))  # type: int
    A = [next(tokens) for _ in range(H) ]  # type: "List[str]"
    solve(H, W, A)

if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
#!/usr/bin/env python3
import sys
from pprint import pprint

def solve(H: int, W: int, A: "List[str]"):
    count = [[H + W] * (W + 2) for _ in range(H + 2)]
    for i in range(H):
        for j in range(W):
            if A[i][j] == '#':
                count[i + 1][j + 1] = 0
            else:
                count[i + 1][j + 1] = min(count[i][j + 1], count[i + 1][j]) + 1

    #pprint(count)
    ret = 0
    for i in range(H, 0, -1):
        for j in range(W, 0, -1):
            count[i][j] = min(count[i][j], min(count[i][j + 1], count[i + 1][j]) + 1)
            ret = max(ret, count[i][j])
    #pprint(count)
    print(ret)
    return


def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    H = int(next(tokens))  # type: int
    W = int(next(tokens))  # type: int
    A = [next(tokens) for _ in range(H) ]  # type: "List[str]"
    solve(H, W, A)

if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys
from pprint import pprint

def solve(H: int, W: int, A: "List[str]"):
    used = [[False] * W for _ in range(H)]
    dx = [1, 0, -1, 0]
    dy = [0, -1, 0, 1]
    s = set()
    for i in range(H):
        for j in range(W):
            if A[i][j] == '#':
                s.add((i, j))
                used[i][j] = True

    ret = 0
    while True:
        nex = set()
        for x, y in s:
            for j in range(4):
                tx = x + dx[j]
                ty = y + dy[j]
                if 0 <= tx < H and 0 <= ty < W and not used[tx][ty]:
                    nex.add((tx, ty))
                    used[tx][ty] = True
        if len(nex) < 1:
            break
        s = nex
        ret += 1

    print(ret)
    return


def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    H = int(next(tokens))  # type: int
    W = int(next(tokens))  # type: int
    A = [next(tokens) for _ in range(H) ]  # type: "List[str]"
    solve(H, W, A)

if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
#!/usr/bin/env python3
import sys
from pprint import pprint

INF = float('inf')

def solve(H: int, W: int, A: "List[str]"):
    count = [[INF] * (W + 2) for _ in range(H + 2)]
    for i in range(H):
        for j in range(W):
            if A[i][j] == '#':
                count[i + 1][j + 1] = 0
            else:
                count[i + 1][j + 1] = min(count[i][j + 1], count[i + 1][j]) + 1

    #pprint(count)
    ret = 0
    for i in range(H, 0, -1):
        for j in range(W, 0, -1):
            count[i][j] = min(count[i][j], min(count[i][j + 1], count[i + 1][j]) + 1)
            ret = max(ret, count[i][j])
    #pprint(count)
    print(ret)
    return


def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    H = int(next(tokens))  # type: int
    W = int(next(tokens))  # type: int
    A = [next(tokens) for _ in range(H) ]  # type: "List[str]"
    solve(H, W, A)

if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys
from pprint import pprint

def solve(H: int, W: int, A: "List[str]"):
    used = [[False] * W for _ in range(H)]
    dx = [1, 0, -1, 0]
    dy = [0, -1, 0, 1]
    s = set()
    for i in range(H):
        for j in range(W):
            if A[i][j] == '#':
                s.add((i, j))
                used[i][j] = True

    ret = 0
    while True:
        nex = set()
        for x, y in s:
            for j in range(4):
                tx = x + dx[j]
                ty = y + dy[j]
                if 0 <= tx < H and 0 <= ty < W and not used[tx][ty]:
                    nex.add((tx, ty))
                    used[tx][ty] = True
        if len(nex) < 1:
            break
        s = nex
        ret += 1

    print(ret)
    return


def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    H = int(next(tokens))  # type: int
    W = int(next(tokens))  # type: int
    A = [next(tokens) for _ in range(H) ]  # type: "List[str]"
    solve(H, W, A)

if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
import sys
from pprint import pprint

INF = float('inf')

def solve(H: int, W: int, A: "List[str]"):
    #corner = [H + W] * 4
    #for i in range(H):
    #    for j in range(W):
    #        if A[i][j] == '#':
    #            corner[0] = min(corner[0], abs(i - 0) + abs(j - 0))
    #            corner[1] = min(corner[1], abs(i - 0) + abs(j - (W - 1)))
    #            corner[2] = min(corner[2], abs(i - (H - 1)) + abs(j - 0))
    #            corner[3] = min(corner[3], abs(i - (H - 1)) + abs(j - (W - 1)))

    count = [[INF] * (W + 2) for _ in range(H + 2)]
    for i in range(H):
        for j in range(W):
            if A[i][j] == '#':
                count[i + 1][j + 1] = 0
            else:
                count[i + 1][j + 1] = min(count[i][j + 1], count[i + 1][j]) + 1

    #pprint(count)
    ret = 0
    for i in range(H, 0, -1):
        for j in range(W, 0, -1):
            count[i][j] = min(count[i][j], min(count[i][j + 1], count[i + 1][j]) + 1)
            ret = max(ret, count[i][j])
    #pprint(count)
    print(ret)
    return


def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    H = int(next(tokens))  # type: int
    W = int(next(tokens))  # type: int
    A = [ next(tokens) for _ in range(W) ]  # type: "List[str]"
    solve(H, W, A)

if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys
from pprint import pprint

def solve(H: int, W: int, A: "List[str]"):
    used = [[False] * W for _ in range(H)]
    dx = [1, 0, -1, 0]
    dy = [0, -1, 0, 1]
    s = set()
    for i in range(H):
        for j in range(W):
            if A[i][j] == '#':
                s.add((i, j))
                used[i][j] = True

    ret = 0
    while True:
        nex = set()
        for x, y in s:
            for j in range(4):
                tx = x + dx[j]
                ty = y + dy[j]
                if 0 <= tx < H and 0 <= ty < W and not used[tx][ty]:
                    nex.add((tx, ty))
                    used[tx][ty] = True
        if len(nex) < 1:
            break
        s = nex
        ret += 1

    print(ret)
    return


def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    H = int(next(tokens))  # type: int
    W = int(next(tokens))  # type: int
    A = [next(tokens) for _ in range(H) ]  # type: "List[str]"
    solve(H, W, A)

if __name__ == '__main__':
    main()