93. 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)

class Solution {
public:
    vector<string> restoreIpAddresses(string s) {
        vector<string> res;
        int len = s.size();

        if(len > 3){
        //[0, i) [i, j) [i + j, k)  [i+ j +k,len)
            for(int i= 1; i < 4 && i < len -2; i++)
                for(int j = 1; j < 4 && i + j < len -1; j++)
                    for(int k = 1; k < 4 && i+ j +k < len; k++){
                        string s1 = s.substr(0, i);
                        string s2 = s.substr(i, j);
                        string s3 = s.substr(i + j, k);
                        string s4 = s.substr(i + j + k);

                        if(isValid(s1) && isValid(s2) && isValid(s3) && isValid(s4)){
                            res.push_back(s1+ "."+ s2+"." + s3+"."+s4);
                        }
                    }
        }
        return res;
    }
private:
    bool isValid(string s){
        if(s.size() > 3 || s.size() == 0 || (s[0]=='0' && s.size() > 1) || atoi(s.c_str())>255)
            return false;
        return true;
    }
};