93. Restore IP Addresses

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

Example:

Input: "25525511135"
Output: ["255.255.11.135", "255.255.111.35"]

AC code:

class Solution {
public:
    vector<string> restoreIpAddresses(string s) {
        vector<string> res;
        helper(0, s, res, "", 0);
        return res;
    }
    
    void helper(int count, string s, vector<string>& res, string ret, int index) {
        if (count > 4) return;
        if (count == 4 && index == s.length())
            res.push_back(ret);
        for (int i = 1; i < 4; i++) {
            if (index+i > s.length()) break;
            string temp = s.substr(index, i);
            if(temp[0] == '0' && temp.length() > 1 || atoi(temp.c_str()) > 255) continue;
            string next = ret + temp;
            helper(count+1, s, res, next+(count == 3 ? "" : "."), index+i);
        }
    }
};

Runtime: 4 ms, faster than 23.68% of C++ online submissions for Restore IP Addresses.