LeetCode Q93: Restore IP Addresses

Given a string containing only digits, restore it by returning all possible valid IP address combinations.

For example:
Given "25525511135",

return ["255.255.11.135", "255.255.111.35"]. (Order does not matter)

Solution, recursion. There are 4 sections in IP address in which there are at most 3 possible situations. Verify whether each case is valid.

Round 2 solution: