题解 | #验证IP地址#

import java.util.*;


public class Solution {
    /**
     * 验证IP地址
     * @param IP string字符串 一个IP地址字符串
     * @return string字符串
     */
    public String solve (String IP) {
        // write code here
        if (IP == null) {
            return "Neither";
        }
        if (isIpV4(IP)) {
            return "IPv4";
        }
        if (isIpV6(IP)) {
            return "IPv6";
        }
        return "Neither";
    }

    private boolean isIpV4(String ip) {
        if (ip.indexOf('.') == -1) {
            return false;
        }
        String[] parts = ip.split("\\.");
        if (parts.length != 4) {
            return false;
        }
        for (int i = 0; i < parts.length; i++) {
            String part = parts[i];
            if (part.length() == 0) {
                return false;
            }
            if (part.length() > 3 || (part.charAt(0) == '0' && part.length() > 1)) {
                return false;
            }
            int num = 0;
            for (int j = 0; j < part.length(); j++) {
                char ch = part.charAt(j);
                if (ch < '0' || ch > '9') {
                    return false;
                }
                num = num * 10 + (int) (ch - '0');
            }
            if (num < 0 || num > 255) {
                return false;
            }
        }
        return true;
    }

    private boolean isIpV6(String ip) {
        if (ip.indexOf(':') == -1) {
            return false;
        }
        String[] parts = ip.split(":", -1);
        if (parts.length != 8) {
            return false;
        }
        for (int i = 0; i < parts.length; i++) {
            String part = parts[i];
            if (part.length() == 0 || part.length() > 4) {
                return false;
            }
            for (int j = 0; j < part.length(); j++) {
                char ch = part.charAt(j);
                if (!((ch >= '0' && ch <= '9') 
                    || (ch >= 'a' && ch <= 'f') 
                    || (ch >= 'A' && ch <= 'F'))) {
                        return false;
                    }
            }
        }
        return true;
    }
}