题解 | #整数与IP地址间的转换#

import java.util.Scanner;

// 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);
        //获取该字符串
        String ss = in.nextLine();
        String[] strs = ss.split("\\.");
        StringBuffer sb = new StringBuffer();
        for (String str : strs) {
            sb.append(adjustToEightBitBinary(Integer.toBinaryString(Integer.parseInt(str)),8));

        }
        int index = 0;
        long result = getTwoToTen(sb.toString());

        String tenTotwo = in.nextLine();

        String tenStr =adjustToEightBitBinary(Long.toBinaryString(Long.parseLong(tenTotwo)),32);
        String s1 = tenStr.substring(tenStr.length() - 8, tenStr.length());
        String s2 = tenStr.substring(tenStr.length() - 16, tenStr.length() - 8);
        String s3 = tenStr.substring(tenStr.length() - 24, tenStr.length() - 16);
        String s4 = tenStr.substring(0, 8);
        StringBuffer result2 = new StringBuffer();
        result2.append(getTwoToTen(s4));
        result2.append(".");
        result2.append(getTwoToTen(s3));
        result2.append(".");
        result2.append(getTwoToTen(s2));
        result2.append(".");
        result2.append(getTwoToTen(s1));

        System.out.println(result);
        System.out.println(result2);

    }


    private static String adjustToEightBitBinary(String binaryNumber,int len) {
        int length = binaryNumber.length();
        if (length < len) {
            int zerosToAdd = len - length;
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < zerosToAdd; i++) {
                sb.append("0");
            }
            sb.append(binaryNumber);
            return sb.toString();
        } else {
            return binaryNumber;
        }
    }

    public static long getTwoToTen(String sb) {
        int index=0;
        long result = 0L;
        for (int i = sb.length()-1; i >=0 ; i--) {
            if (sb.charAt(i) == '1') {
                result += (long)Math.pow(2, index);
            }
            index++;
        }
        return result;
    }



}

这道题不难但是要注意的点有以下几个

1、如何将Int或者是Long转成二进制，需要记住方法Integer.toBinaryString，Long.toBinaryString

2、方法1转换过来的，不一定是8位，需要写方法补齐八位，切记切记！！！！

时间复杂度：O(n)

空间复杂度：O(n)