问了几级才拿到

按委派树，解析 img.cdn.net 问过几级服务器？

贯穿本节的委派树（本地 resolver 迭代式逐级问；判题机不联网，这是它的确定模型）：根 知道每个顶级域（TLD，如 com）交给哪台服务器，TLD 服务器知道每个域（如 example.com）的权威服务器是谁，权威服务器上才有最终的 IP。chain(名字) 交回一路问过的服务器名（从根到权威），levels 是问过几级，resolve 交回 IP。

DELEG = {
    "根": {"com": "com服务器", "net": "net服务器", "org": "org服务器"},
    "com服务器": {"example.com": "example权威", "shop.com": "shop权威"},
    "net服务器": {"cdn.net": "cdn权威"},
    "org服务器": {"wiki.org": "wiki权威"},
}
AUTH = {
    "example权威": {"www.example.com": "93.184.216.34", "mail.example.com": "93.184.216.35"},
    "shop权威": {"www.shop.com": "10.0.0.9"},
    "cdn权威": {"img.cdn.net": "203.0.113.7"},
    "wiki权威": {"wiki.org": "198.51.100.2"},
}


def tld_of(name):
    return name.rsplit(".", 1)[-1]


def zone_of(name):
    # 权威区（去掉最左的主机名那一段，如 www.example.com → example.com）
    parts = name.split(".")
    return ".".join(parts[-2:])


def chain(name):
    # 迭代式解析：本地 resolver 从根问起，一级级拿到「去问下一个」的指引，返回问过的服务器名
    seq = ["根"]
    tld = tld_of(name)
    tsrv = DELEG["根"][tld]
    seq.append(tsrv)
    asrv = DELEG[tsrv][zone_of(name)]
    seq.append(asrv)
    return seq


def levels(name):
    # 问了几级服务器才拿到答案
    return len(chain(name))


def resolve(name):
    asrv = chain(name)[-1]
    return AUTH[asrv][name]

print(levels("img.cdn.net"))