算法

(模拟) $O(n)$

按题意模拟一下即可。

Java 代码

class Solution {
    public String validIPAddress(String IP) {
        String ip = IP.toLowerCase();
        String[] ip4 = ip.split("\\.", -1);
        if (ip4.length == 4) return checkIPv4(ip4);
        String[] ip6 = ip.split(":", -1);
        if (ip6.length == 8) return checkIPv6(ip6);
        return "Neither";
    }

    public String checkIPv4(String[] ip) {
        for (String s : ip) {
            int n = s.length();
            // check length
            if (n == 0 || n > 3) return "Neither";
            // check digits
            for (int i = 0; i < n; ++i) {
                if (!Character.isDigit(s.charAt(i))) return "Neither";
            }
            // check range
            int num = Integer.valueOf(s);
            if (num < 0 || num > 255) return "Neither";
            // check leading zero
            if (String.valueOf(num).length() < n)  return "Neither";
        }
        return "IPv4";
    }

    public String checkIPv6(String[] ip) {
        for (String s : ip) {
            int n = s.length();
            if (n == 0 || n > 4) return "Neither";
            // check digits
            for (int i = 0; i < n; ++i) {
                char c = s.charAt(i);
                if (c >= '0' && c <= '9' || c >= 'a' && c <= 'f') continue;
                return "Neither";
            }
        }
        return "IPv6";
    }
}