题解 | #判断两个IP是否属于同一子网#

import java.util.Scanner;
import java.util.*;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        List<String> s = new ArrayList<>();
        while (in.hasNextLine()) {
            s.add(in.nextLine());
        }
        //分组循环，3行为一组
        for (int i = 0; i < s.size(); i++) {
            if ((i + 1) % 3 == 0) {
                int num = judge(s.get(i - 2), s.get(i - 1), s.get(i));
                System.out.println(num);
            }
        }


    }

    public static int judge(String netmask, String ip1, String ip2) {
        int result = 110;
        //判断输入合法性(数字在[0,255])
        boolean flag = true;
        for (int i = 0; i < 4; i++) {
            flag = judgeNum(netmask.split("\\.")[i]);

            if (flag == false) {
                return 1;
            }
            flag = judgeNum(ip1.split("\\.")[i]);
            if (flag == false) {
                return 1;
            }
            flag = judgeNum(ip2.split("\\.")[i]);
            if (flag == false) {
                return 1;
            }
        }
	  	//判断子网掩码正确性，必须是连续的1和0
        boolean flag1 = true;
        for (int i = 0; i < 4; i++) {
            String j = Integer.toBinaryString(Integer.parseInt(netmask.split("\\.")[i])|1<<8).substring(1);
            if (flag1 == true) {
                if (j.contains("0")) {
                    flag1 = false;
                    int index = j.indexOf("0",0);
                    if(j.substring(index).contains("1")){
                        return 1;
                    }
                }
            }else{
                if(j.contains("1")){
                    return 1;
                }
            }

        }
        //判断是否属于同一子网
        if (toBinary(netmask, ip1).toString().equals(toBinary(netmask,
                ip2).toString())) {
            return 0;
        }
        return 2;
    }

    public static StringBuffer toBinary(String s, String t) {
        String[] sArray = s.split("\\.");
        String[] tArray = t.split("\\.");
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < 4; i++) {
            int f =  Integer.parseInt(sArray[i]) & Integer.parseInt(tArray[i]);
            sb.append(Integer.toBinaryString(f | 1 << 8));
        }
        return sb;
    }
    public static boolean judgeNum(String s) {
        if (Integer.parseInt(s) >= 0 && Integer.parseInt(s) < 256) {
            return true;
        }
        return false;
    }
}