题解 | #判断两个IP是否属于同一子网#

import java.math.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import java.util.stream.*;
import java.util.regex.*;
import java.util.function.*;
import java.net.*;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

        while (in.hasNextLine()) {
            String mask = in.nextLine();
            String ip1 = in.nextLine();
            String ip2 = in.nextLine();

            if (!isMaskIleagl(mask) || !isIPIleagl(ip1) || !isIPIleagl(ip2)) {
                System.out.println(1);
                continue;
            }

            if (maskIP(mask, ip1).equals(maskIP(mask, ip2))) {
                System.out.println(0);
            } else {
                System.out.println(2);
            }
        }

    }

    static String maskIP(String mask, String ip) {
        List<String> list = new ArrayList<>();
        String[] m = mask.split("\\.");
        String[] p = ip.split("\\.");

        for (int i = 0; i < 4; i++) {
            int and =  Integer.parseInt(m[i]) & Integer.parseInt(p[i]);
            list.add(String.valueOf( and ));
        }
        return String.join(".", list);
    }

    static boolean isIPIleagl(String ip) {
        try {
            InetAddress address = InetAddress.getByName(ip);
            return address.getHostAddress().equals(ip);
        } catch (UnknownHostException e) {
            return false;
        }
    }

    static boolean isMaskIleagl(String mask) {
        if (!isIPIleagl(mask)) {
            return false;
        }

        String[] nums = mask.split("\\.");
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < 4; i++) {
            String bStr = Integer.toBinaryString(Integer.parseInt(nums[i]));
            // 技巧！！
            bStr = String.join("", Collections.nCopies(8 - bStr.length(), "0")) + bStr;
            sb.append(bStr);
        }

        // 符合先 1 后 0
        boolean has0 = false;
        for (int i = 0; i < sb.length(); i++) {
            if ('0' == sb.charAt(i)) {
                has0 = true;
            }
            if (has0 && '1' == sb.charAt(i)) {
                return false;
            }
        }
        return true;
    }
}