https://leetcode.com/problems/validate-ip-address/

validate-ip-address.java

class Solution {
    public String validIPAddress(String IP) {
        String[] ipv4 = IP.split("\\.");
        String[] ipv6 = IP.split("\\:");
        
        if (ipv4.length == 4) {
            if (countSep(IP, '.') > 3) {
                return "Neither";
            }
            for (int i = 0; i < 4; i++) {
                if (!isIPV4(ipv4[i])) {
                    return "Neither";
                }
            }
            return "IPv4";
        }
        
        if (ipv6.length == 8) {
            if (countSep(IP, ':') > 7) {
                return "Neither";
            } 
            for (int i = 0; i < 8; i++) {
                if (!isIPV6(ipv6[i])) {
                    return "Neither";
                }
            }
            return "IPv6";
        }
        
        return "Neither";
    }
    
    private int countSep(String s, char c) {
        int cnt = 0;
        for (char ch : s.toCharArray()) {
            if (ch == c) {
                cnt += 1;
            }
        }
        return cnt;
    }
    
    private boolean isIPV4(String s) {
        if (s == null || s.length() == 0) {
            return false;
        }
        int len = s.length();
        if (len > 3 || len > 1 && s.charAt(0) == '0') {
            return false; 
        }
        int num = 0;
        for (int i = 0; i < len; i++) {
            char curr = s.charAt(i);
            if (curr < '0' || curr > '9') {
                return false;
            }
            num = num * 10 + (s.charAt(i) - '0');
            if (num > 255) {
                return false;
            }
        }
        return num >= 0 && num < 256;
    }
    
    
    private boolean isIPV6(String s) {
        if (s == null || s.length() == 0) {
            return false;
        }
        int len = s.length();
        if (len > 4) {
            return false;
        }
        for (int i = 0; i < len; i++) {
            if (!isValid(s.charAt(i))) {
                return false;
            }
        }
        return true;
    }
    
    private boolean isValid(char c) {
        return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f') || (c >= 'A' && c <= 'F');
    }
    
}