题目要求

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.

校验该字符串是IPV4地址还是IPV6地址还是二者都不是。
IPV4地址通过小数点分割为4个部分，每个部分都是0～255之间的正整数，且不能包含开头的0，如013
IPV6地址通过冒号分割为8个部分，允许以0作为开头，但是每个部分不能用空代替0。

思路和代码

其实这题考验的是测试思维，即如何能够把所有的边界情况考虑清楚。这里我简单列举一下几个边界场景

1. IPV4地址中小数点个数超过3个
2. IPV4地址中小数点分割开来的每个部分中包含非整数值
3. IPV4地址中小数点分割开来的每个部分数值不再0～255之间
4. IPV6地址中冒号个数超过7个
5. IPV6地址中分割开来的每个部分包含空值
6. IPV6地址中分割出来的每个部分长度不是1～4
7. IPV6地址中分割出来的每个部分包含非16进制数中包含的内容

代码如下：

    private static final String IPV4 = "IPv4";
    private static final String IPV6 = "IPv6";
    private static final String NEITHER = "Neither";

    public String validIPAddress(String IP) {
        if (IP==null ||IP.isEmpty()) {return NEITHER;}
        if (IP.contains(".")) {
            return checkIPv4(IP);
        }else if (IP.contains(":")) {
            return checkIPv6(IP);
        }else {
            return NEITHER;
        }
    }

    private String checkIPv4(String IP) {
        if (IP.startsWith(".") || IP.endsWith(".")) {return NEITHER;}
        String[] addressParts = IP.split("\\.");
        if (addressParts.length != 4) {
            return NEITHER;
        }
        for (String addressPart :addressParts) {
            if (addressPart.length() <= 0
                    || addressPart.length() > 3
                    || addressPart.startsWith("0") && addressPart.length() > 1 ) {
                return NEITHER;
            }
            int value = 0;
            for (char c : addressPart.toCharArray()) {
                if (c < '0' || c > '9') {
                    return NEITHER;
                }
                value = value * 10 + (c - '0');
            }
            if (value < 0 || value > 255) {
                return NEITHER;
            }
        }
        return IPV4;
    }

    private String checkIPv6(String IP) {
        if (IP.startsWith(":") || IP.endsWith(":")) {return NEITHER;}
        String[] addressParts = IP.split(":");
        if (addressParts.length != 8) {
            return NEITHER;
        }
        for (String addressPart : addressParts) {
            if (addressPart.length() <= 0 || addressPart.length() > 4) {
                return NEITHER;
            }
            for (char c : addressPart.toCharArray()) {
                char lowerChar = Character.toLowerCase(c);
                if (!((lowerChar >= '0' && lowerChar <= '9') || (lowerChar >= 'a' && lowerChar <= 'f'))) {
                    return NEITHER;
                }
            }
        }
        return IPV6;
    }