补全：TTL 用完就停

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（这一节多了逐跳模拟：NODES 里每个节点有接口地址和一张小路由表；next_hop 在某节点上查表，owner 找哪个节点持有某个地址，trace 从起点一跳跳走到目标，交回 (路径, 结果)。）

补全 trace_ttl(nodes, start, target, ttl)：和 trace 一样，但每走一跳 ttl 减一，减到 0 还没到就交回「TTL耗尽」和当前路径。用 ttl=2 走一次。

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def to_int(ip):
    a, b, c, d = [int(x) for x in ip.split(".")]
    return (a << 24) | (b << 16) | (c << 8) | d

def mask(prefix):
    return (0xFFFFFFFF << (32 - prefix)) & 0xFFFFFFFF

def contains(cidr, ip):
    net, p = cidr.split("/")
    return to_int(net) & mask(int(p)) == to_int(ip) & mask(int(p))

NODES = {
    "主机": (["192.168.10.5/24"], [("0.0.0.0/0", "192.168.10.1")]),
    "R1": (["192.168.10.1/24", "10.0.1.1/24"], [("172.16.0.0/16", "10.0.1.2"), ("0.0.0.0/0", "10.0.1.2")]),
    "R2": (["10.0.1.2/24", "10.0.2.1/24"], [("172.16.0.0/16", "10.0.2.2"), ("192.168.10.0/24", "10.0.1.1")]),
    "R3": (["10.0.2.2/24", "172.16.0.1/24"], [("192.168.10.0/24", "10.0.2.1")]),
    "目标": (["172.16.0.9/24"], [("0.0.0.0/0", "172.16.0.1")]),
}

def owner(nodes, ip):
    for name, (addrs, _) in nodes.items():
        if any(a.split("/")[0] == ip for a in addrs):
            return name
    return None

def next_hop(nodes, name, target):
    addrs, table = nodes[name]
    for a in addrs:
        if contains(a, target):
            return "直连"
    best = None
    for cidr, via in table:
        if contains(cidr, target):
            plen = int(cidr.split("/")[1])
            if best is None or plen > best[0]:
                best = (plen, via)
    return best[1] if best else None

def trace(nodes, start, target, ttl=16):
    path = [start]
    cur = start
    while ttl > 0:
        hop = next_hop(nodes, cur, target)
        if hop is None:
            return path, "无路可走"
        if hop == "直连":
            dst = owner(nodes, target)
            if dst is None:
                return path, "同段但无人应答"
            path.append(dst)
            return path, "到达"
        cur = owner(nodes, hop)
        if cur is None:
            return path, "下一跳不存在"
        path.append(cur)
        ttl -= 1
    return path, "TTL耗尽"

def trace_ttl(nodes, start, target, ttl):
    path = [start]
    cur = start
    # TODO: while ttl > 0 循环，同 trace；循环结束返回 path, "TTL耗尽"
    return path, "?"

p, r = trace_ttl(NODES, "主机", "172.16.0.9", 2)
q, s = trace_ttl(NODES, "主机", "172.16.0.9", 4)
print(",".join(p) + "/" + r + "|" + ",".join(q) + "/" + s)

本次输入：

输出：

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补全：TTL 用完就停

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