468. Validate IP Address - Detailed Explanation

Problem Statement

Given a string representing an IP address, determine whether it is a valid IPv4 or IPv6 address or neither. An IPv4 address consists of four decimal numbers separated by dots. Each number must be between 0 and 255 and cannot have leading zeros (except for the number 0 itself). An IPv6 address consists of eight groups separated by colons. Each group is a hexadecimal number (digits 0-9 and letters a-f/A-F) and can contain from 1 to 4 hexadecimal digits.

Example 1
Input: "172.16.254.1"
Output: "IPv4"
Explanation: Each part (172, 16, 254, 1) is in the correct range and properly formatted.

Example 2
Input: "2001:0db8:85a3:0:0:8A2E:0370:7334"
Output: "IPv6"
Explanation: There are eight groups, and every group contains valid hexadecimal digits.

Example 3
Input: "256.256.256.256"
Output: "Neither"
Explanation: Each number in an IPv4 address must be between 0 and 255; here, 256 is out of range.

Constraints:
The input string is non-empty and contains only alphanumeric characters along with the characters '.' and ':'.

Hints

Hint 1: Check the separator in the string. If it contains dots ('.'), treat it as an IPv4 candidate; if it contains colons (':'), treat it as an IPv6 candidate.

Hint 2: For IPv4, validate that each segment is non-empty, contains only digits, has no leading zeros (unless the segment is exactly "0"), and represents a number between 0 and 255. For IPv6, ensure that each segment has between 1 and 4 characters and that every character is a valid hexadecimal digit (0-9, a-f, A-F).

Brute Force Approach

Idea: Split the input string by the expected separator ('.' for IPv4 and ':' for IPv6) and manually check each part against the required rules.
Steps:

• IPv4 Check:
  • Split the string by "." and ensure there are exactly 4 parts.
  • For each part, check that it is non-empty, contains only digits, does not have leading zeros (unless it is "0"), and its integer value is between 0 and 255.
• IPv6 Check:
  • Split the string by ":" and ensure there are exactly 8 parts.
  • For each part, ensure its length is between 1 and 4 and that every character is a valid hexadecimal digit.

Pros: Simple and easy to understand.
Cons: Involves multiple conditional checks, but overall time complexity remains linear.

Optimal Approach

Idea: Use the same splitting strategy but streamline the validation process. Optionally, regular expressions can be used for pattern matching; however, the explicit approach is often clearer for junior engineers.

Steps:

• Determine the type by checking for a dot or a colon.
• For IPv4, validate digit-only segments and use error handling while converting to numbers.
• For IPv6, use a set of valid hexadecimal characters for quick lookup and validate each segment’s length and characters.

Pros: Clear logic with early termination on failure.
Cons: Regex might reduce code length but can compromise readability for beginners.

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