原题 https://www.codewars.com/kata/int32-to-ipv4/train/java

Take the following IPv4 address: 128.32.10.1

This address has 4 octets where each octet is a single byte (or 8 bits).

  • 1st octet 128 has the binary representation: 10000000
  • 2nd octet 32 has the binary representation: 00100000
  • 3rd octet 10 has the binary representation: 00001010
  • 4th octet 1 has the binary representation: 00000001

So 128.32.10.1 == 10000000.00100000.00001010.00000001

Because the above IP address has 32 bits, we can represent it as the unsigned 32 bit number: 2149583361

Complete the function that takes an unsigned 32 bit number and returns a string representation of its IPv4 address.

  1. 2149583361 ==> "128.32.10.1"
  2. 32         ==> "0.0.0.32"
  3. 0          ==> "0.0.0.0"

 

  1. public class Kata {
  2.     public static String longToIP(long ip) {
  3.         //1. translate the ip to binary representation
  4.         String str = "";
  5.         if (ip == 0) {
  6.             str = ip + "";
  7.         } else {
  8.             while (ip != 0) {
  9.                 str = ip % 2 + str;
  10.                 ip = ip / 2;
  11.             }
  12.         }
  14.         //2. if the binary string short than 32 bit, then add "0" in front
  15.         while (str.length() != 32) {
  16.             str = "0" + str;
  17.         }
  19.         String result = "";
  20.         //3. truncate the str to four items
  21.         for (int i = 0; i < 4; i++) {
  22.             String partStr = str.substring(i * 8, 8 * (i + 1));
  23.             //4. translate every item to decimal number
  24.             int bi = Integer.parseInt(partStr, 2);
  25.             if (i == 3) {
  26.                 result += bi + "";
  27.             } else {
  28.                 result += bi + ".";
  29.             }
  30.         }
  31.         return result;
  32.     }
  33. }

 

1.

  1. public class Kata {
  2.   public static String longToIP(long ip) {
  3.     return String.format("%d.%d.%d.%d", ip>>>24, (ip>>16)&0xff, (ip>>8)&0xff, ip&0xff);
  4.   }
  5. }

2.

  1. public class Kata {
  2.   public static String longToIP(long ip) {
  3.     String[] e = new String[4];
  4.     int i = 4;
  5.     while (i-- != 0) {
  6.       e[i] = "" + (ip % 256);
  7.       ip /= 256;
  8.     }
  9.     return String.join(".",e);
  10.   }
  11. }

3.

  1. public class Kata {
  2.   public static String longToIP(long ip) {
  3.     return IntStream.rangeClosed(1, 4)
  4.       .mapToObj(i -> String.valueOf(ip >> (32 - 8 * i) & 255))
  5.       .collect(Collectors.joining("."));
  6.   }
  7. }

 

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