题解 | #判断两个IP是否属于同一子网#

import java.util.Scanner;

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

    while(sc.hasNext()) {
        String mask = sc.nextLine();
        String ip_a = sc.nextLine();
        String ip_b = sc.nextLine();
        
        // Check if the mask is illegal
        if(judgeMask(mask) == 1) {
        // Check ip address is illegal
            if(judgeIpAddr(ip_a) == 1 && judgeIpAddr(ip_b) == 1) {
                 // Convert to long date type and compare those ips
                 Long maskq = ipConvertLong(mask);
                 Long ipA = ipConvertLong(ip_a)&maskq;
                 Long ipB = ipConvertLong(ip_b)&maskq;
                 if(ipA.equals(ipB)) {
                    System.out.println(0);
                 } else {
                    System.out.println(2);
                 }
            } else {
                System.out.println(1);
            }
        } else {
            System.out.println(1);
        }
    }
} 

private static int judgeMask(String s) {
    String[] ss = s.split("\\.");
    if(ss.length != 4) {
        return 0;
    }
    
    for(String str: ss) {
        if(Integer.parseInt(str) > 255 || Integer.parseInt(str) < 0) {
            return 0;
        }
    }
    
    for(int i = 0; i < 3; i++) {
        if(Integer.parseInt(ss[i]) > 255 || Integer.parseInt(ss[i]) < 0) {
            return 0;
        }
        if( i == 0 && Integer.parseInt(ss[i]) < 255) {
            if(Integer.parseInt(ss[i+1]) > 0) {
                return 0;
            } else if(Integer.parseInt(ss[i+2]) > 0) {
                return 0;
            } else if(Integer.parseInt(ss[i+3]) > 0) {
                return 0;
            }
        } else if( i == 1 && Integer.parseInt(ss[i]) < 255) {
            if(Integer.parseInt(ss[i+1]) > 0) {
                return 0;
            } else if(Integer.parseInt(ss[i+2]) > 0) {
                return 0;
            }
        } else if( i == 2 && Integer.parseInt(ss[i]) < 255) {
            if(Integer.parseInt(ss[i+1]) > 0) {
                return 0;
            }
        }
        
    }
    
    return 1;
}

private static int judgeIpAddr(String s) {
    String[] ss = s.split("\\.");
    if(ss.length != 4) {
        return 0;
    }
    for(String str: ss) {
        if(Integer.parseInt(str) > 255 || Integer.parseInt(str) < 0) {
            return 0;
        }
    }
    return 1;
}

private static Long ipConvertLong(String ip) {
    Long ipt= 0L;
    for(int i = 0; i < 4; i++) {
        ipt = ipt << 8 | Integer.parseInt(ip.split("\\.")[i]);
    }
    return ipt;
}

}