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.

一刷
题解：
主要是对于格式的理解问题：
对于ipv4

• without leading zero
• 所有的character都是digit
  corner case:
  -0
  if(parsedInt==0 && token.charAt(0)!='0') return false;

对于ipv6

• group的长度是8，每个group中长度为4的16进制数字
• without leading zero
• 忽略大小写差异
• 多个相邻的为全0的group， 比如:0:0:可以简写为::

注意string这个object几个好用的方法：

startwith
parseInt(throw NumberFormatException)

public String validIPAddress(String IP) {
    if(isValidIPv4(IP)) return "IPv4";
    else if(isValidIPv6(IP)) return "IPv6";
    else return "Neither";
}

public boolean isValidIPv4(String ip) {
    if(ip.length()<7) return false;
    if(ip.charAt(0)=='.') return false;
    if(ip.charAt(ip.length()-1)=='.') return false;
    String[] tokens = ip.split("\\.");
    if(tokens.length!=4) return false;
    for(String token:tokens) {
        if(!isValidIPv4Token(token)) return false;
    }
    return true;
}
public boolean isValidIPv4Token(String token) {
    if(token.startsWith("0") && token.length()>1) return false;
    try {
        int parsedInt = Integer.parseInt(token);
        if(parsedInt<0 || parsedInt>255) return false;
        if(parsedInt==0 && token.charAt(0)!='0') return false;
    } catch(NumberFormatException nfe) {
        return false;
    }
    return true;
}
    
public boolean isValidIPv6(String ip) {
    if(ip.length()<15) return false;
    if(ip.charAt(0)==':') return false;
    if(ip.charAt(ip.length()-1)==':') return false;
    String[] tokens = ip.split(":");
    if(tokens.length!=8) return false;
    for(String token: tokens) {
        if(!isValidIPv6Token(token)) return false;
    }
    return true;
}
public boolean isValidIPv6Token(String token) {
    if(token.length()>4 || token.length()==0) return false;
    char[] chars = token.toCharArray();
    for(char c:chars) {
        boolean isDigit = c>=48 && c<=57;
        boolean isUppercaseAF = c>=65 && c<=70;
        boolean isLowerCaseAF = c>=97 && c<=102;
        if(!(isDigit || isUppercaseAF || isLowerCaseAF)) 
            return false;
    }
    return true;
}