题解 | #HJ33 整数与IP地址间的转换#

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        while (in.hasNextLine()) {
            String s = in.nextLine();
            if (s.contains(".")) {
                System.out.println(ipToNum(s));
            } else {
                System.out.println(NumToIp(s));
            }
        }
    }

    //ip -> num
    public static long  ipToNum(String ip) {
        String[] split = ip.split("\\.");
        long result = 0;
        //将数字做位运算再累加起来，注意使用long
        for (int i = 0; i < split.length; i++) {
            long num = Long.parseLong(split[i]);
            result += num << (split.length - 1 - i) * 8;
        }
        return result;
    }

    //num -> ip
    public static String NumToIp(String s) {
        long num = Long.parseLong(s);
        StringBuffer sb = new StringBuffer();
        for (int i = 3; i >= 0; i--) {
            sb.append(num / (1 << 8 * i) % 256);
            sb.append(".");
        }
        String result = sb.toString();
        return result.substring(0, result.length() - 1);
    }
}