Validate IP Address - Video Solutions

Problem Overview: Given a string queryIP, determine whether it represents a valid IPv4 address, a valid IPv6 address, or neither. The validation rules differ for each format: IPv4 contains four decimal numbers separated by dots, while IPv6 contains eight hexadecimal groups separated by colons.

Approach 1: Split and Validate (O(n) time, O(1) space)

This approach parses the input string using delimiter-based splitting. First check whether the string contains '.' or ':'. For IPv4, split by dots and verify exactly four segments. Each segment must contain only digits, cannot have leading zeros unless the value is exactly 0, and must fall within the range 0–255. For IPv6, split by colons and verify exactly eight segments. Each segment must contain 1–4 hexadecimal characters (0-9, a-f, A-F). Iterate through each segment and validate character-by-character.

This method works well because the format rules are strict and easy to enforce with simple string checks. Operations include split, length validation, numeric parsing, and character validation. Since the input length is bounded (maximum 39 characters for IPv6), the algorithm runs in linear time relative to the string length. Most interview solutions use this strategy because it clearly demonstrates understanding of string processing and input validation.

Approach 2: Regular Expression Validation (O(n) time, O(1) space)

Another option uses carefully constructed regular expressions to match valid IPv4 and IPv6 formats. One regex pattern enforces IPv4 rules: four numeric groups separated by dots with values restricted to 0–255. Another pattern enforces IPv6 rules: eight groups of 1–4 hexadecimal characters separated by colons. Run the string against both patterns; if the IPv4 pattern matches return "IPv4", if the IPv6 pattern matches return "IPv6", otherwise return "Neither".

Regex compresses the validation logic into a single pattern match. The tradeoff is readability—complex numeric range checks make the expression harder to maintain. Still, regex engines scan the string only once, giving linear time complexity. This method is common in production codebases where regular expression validation already exists.

Recommended for interviews: The split-and-validate approach. It shows clear reasoning about parsing, numeric constraints, and edge cases like leading zeros or invalid hexadecimal characters. Regex demonstrates familiarity with pattern matching but hides the validation logic, so interviewers often prefer the explicit string parsing approach using string operations.