Validate and restore IPv4 addresses

Design and implement the following IPv4 tasks.

a) Validate dotted IPv4: Write a function isValidIPv4 (s) that returns true if s is a valid dotted-decimal IPv4 address and false otherwise. Requirements:

• Exactly four decimal octets separated by three dots.
• Each octet is digits only and represents an integer in [0, 255].
• No leading zeros unless the octet is exactly "0".
• No plus/minus signs, whitespace, or extra characters. Examples: "255.255.11.135" -> true; "-1.255.11.135" -> false; "01.2.3.4" -> false; "256.0.0.1" -> false.

b) Validate undelimited IPv4 possibility: Given a string s consisting only of digits (no dots), return true if it can be segmented into a valid IPv4 by inserting three dots under the same rules as (a); otherwise return false. Examples: "25525511135" -> true (e.g., 255.255.11. 135); "0002555" -> true (e.g., 0.0.255. 55); "123" -> false.

c) Restore all valid IPv4 addresses: Given a string s of digits, return all valid IPv4 addresses formed by inserting three dots under the rules above. Return in any order without duplicates. Example: "25525511135" -> ["255.255.11.135", "255.255.111.35"].

Constraints for (b) and (c): 1 <= s.length <= 12. Aim for an efficient solution (typical approach uses DFS/backtracking).