Validate IP Address

Write a function to check whether an input string is a valid IPv4 address or IPv6 address or neither.

IPv4 addresses are canonically represented in dot-decimal notation, which consists of four decimal numbers, each ranging from 0 to 255, separated by dots ("."), e.g.,172.16.254.1;

Besides, leading zeros in the IPv4 is invalid. For example, the address 172.16.254.01 is invalid.

IPv6 addresses are represented as eight groups of four hexadecimal digits, each group representing 16 bits. The groups are separated by colons (":"). For example, the address 2001:0db8:85a3:0000:0000:8a2e:0370:7334 is a valid one. Also, we could omit some leading zeros among four hexadecimal digits and some low-case characters in the address to upper-case ones, so 2001:db8:85a3:0:0:8A2E:0370:7334 is also a valid IPv6 address(Omit leading zeros and using upper cases).

However, we don't replace a consecutive group of zero value with a single empty group using two consecutive colons (::) to pursue simplicity. For example, 2001:0db8:85a3::8A2E:0370:7334 is an invalid IPv6 address.

Besides, extra leading zeros in the IPv6 is also invalid. For example, the address 02001:0db8:85a3:0000:0000:8a2e:0370:7334 is invalid.

Note: You may assume there is no extra space or special characters in the input string.

Example 1:

Input: "172.16.254.1"

Output: "IPv4"

Explanation: This is a valid IPv4 address, return "IPv4".

 

Example 2:

Input: "2001:0db8:85a3:0:0:8A2E:0370:7334"

Output: "IPv6"

Explanation: This is a valid IPv6 address, return "IPv6".

 

Example 3:

Input: "256.256.256.256"

Output: "Neither"

Explanation: This is neither a IPv4 address nor a IPv6 address.

思路：就是一個按照規律寫代碼的題目，注意幾個細節：split("\\.",-1) -1表示apply as much as possible，就是後面有....也要split

IP.chars().filter(ch -> ch == '.').count() == 3, 這個是新的java 8的寫法，很簡短；

判斷01 != 1，方法是 String.valueOf(Integer.valueOf(s)).equals(s), 01 != 1;

 Integer.parseInt(s, 16) ，這個是16進制轉換。說白了，這個題目，考察不了什麼，全是對API的考察沒有太多的實際意義；

class Solution {
    public String validIPAddress(String IP) {
        String[] ipv4 = IP.split("\\.", -1);
        String[] ipv6 = IP.split("\\:", -1);
        if(IP.chars().filter(ch -> ch == '.').count() == 3) {
            for(String s: ipv4) {
                if(isIPv4(s)) {
                    continue;
                } else {
                    return "Neither";
                }
            }
            return "IPv4";
        }
        
        if(IP.chars().filter(ch -> ch == ':').count() == 7) {
            for(String s: ipv6) {
                if(isIPv6(s)) {
                    continue;
                } else {
                    return "Neither";
                }
            }
            return "IPv6";
        }
        
        return "Neither";
    }
    
    private boolean isIPv4(String s) {
        try {
            return String.valueOf(Integer.valueOf(s)).equals(s) && // 判斷 01 is invalid, 1 != 01;
                    0 <= Integer.parseInt(s) && Integer.parseInt(s) <= 255;
        } catch (NumberFormatException e) {
            return false;
        }
    }
    
    private boolean isIPv6(String s) {
        if(s.length() > 4) {
            return false;
        }
        try {
            return Integer.parseInt(s, 16) >= 0 && s.charAt(0) != '-';
        } catch (NumberFormatException e) {
            return false;
        }
    }
}