题解 | 识别有效的IP地址和掩码并进行分类统计

import sys

def parse_ipv4(s: str):
    parts = s.split('.')
    if len(parts) != 4:
        return None
    nums = []
    for p in parts:
        if p == "" or not p.isdigit():
            return None
        v = int(p)
        if v < 0 or v > 255:
            return None
        nums.append(v)
    return nums

def mask_valid(mask_parts):
    # mask_parts 已经是 4 个 0~255 的整数
    bits = ''.join(f'{x:08b}' for x in mask_parts)  # 32-bit string
    if bits == '0' * 32 or bits == '1' * 32:
        return False

    # 必须是 若干个1 后跟 若干个0
    k = bits.find('0')  # 第一次出现0的位置
    if k == -1:  # 没有0（全1已排除），理论不会到这
        return False
    return (set(bits[:k]) == {'1'}) and (set(bits[k:]) == {'0'})

def is_private(ip):
    a, b, c, d = ip
    if a == 10:
        return True
    if a == 172 and 16 <= b <= 31:
        return True
    if a == 192 and b == 168:
        return True
    return False

def classify(ip):
    a = ip[0]
    if 1 <= a <= 127:
        return 'A'
    if 128 <= a <= 191:
        return 'B'
    if 192 <= a <= 223:
        return 'C'
    if 224 <= a <= 239:
        return 'D'
    if 240 <= a <= 255:
        return 'E'
    return None

def main():
    A = B = C = D = E = ERR = PRI = 0

    for line in sys.stdin:
        line = line.strip()
        if not line:
            continue
        if '~' not in line:
            # 格式都不对，算错误（但题目一般不会给这种）
            ERR += 1
            continue

        ip_s, mask_s = line.split('~', 1)

        ip = parse_ipv4(ip_s)
        # 跳过规则：只有在 IP 能解析出第一段时才能判断 0/127
        if ip is not None and ip[0] in (0, 127):
            continue

        mask_parts = parse_ipv4(mask_s)

        # 先判 IP/mask 格式合法
        if ip is None or mask_parts is None:
            ERR += 1
            continue

        # 再判掩码规则合法
        if not mask_valid(mask_parts):
            ERR += 1
            continue

        # 到这里：IP 和 MASK 都合法，开始分类
        cls = classify(ip)
        if cls == 'A':
            A += 1
        elif cls == 'B':
            B += 1
        elif cls == 'C':
            C += 1
        elif cls == 'D':
            D += 1
        elif cls == 'E':
            E += 1

        if is_private(ip):
            PRI += 1

    print(A, B, C, D, E, ERR, PRI)

if __name__ == "__main__":
    main()