题解 | #整数与IP地址间的转换#

import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        /**********************************************************************************/
        // IPv4 转 整数
        String[] strs = scan.nextLine().split("\\.");
        StringBuffer sb = new StringBuffer("");
        for (String str : strs) {
            sb.append(DecimalToBinary(str, 8));
        }
        System.out.println(BinaryToDecimal(new String(sb), 31));
        /**********************************************************************************/
        // 整数 转 IPv4
        String decimal = scan.nextLine();
        String binary = DecimalToBinary(decimal, 32);
        System.out.println(BinaryToIPv4(binary));
    }
    public static String DecimalToBinary(String str, int k) {
        long num = Long.valueOf(str);
        StringBuffer sb = new StringBuffer("");
        while (0 != num) {
            sb.append(num % 2);
            num /= 2;
        }
        while (sb.length() != k) {
            sb.append("0");
        }
        sb.reverse();
        return new String(sb);
    }
    public static long BinaryToDecimal(String str, int k) {
        long ans = 0l;
        String[] strs = str.split("");
        for (int i = strs.length - 1; i > -1; i--) {
            if (Integer.valueOf(strs[i]) == 1) {
                ans += Math.pow(2, k - i);
            }
        }
        return ans;
    }
    public static String BinaryToIPv4(String str) {
        StringBuffer sb = new StringBuffer("");
        for (int splitIndex = 8; splitIndex <= 32; splitIndex += 8) {
            String tmp = str.substring(splitIndex - 8, splitIndex);
            if (splitIndex == 32) {
                sb.append(BinaryToDecimal(tmp, 7));
                break;
            }
            tmp = BinaryToDecimal(tmp, 7) + ".";
            sb.append(tmp);
        }
        return new String(sb);
    }
}