LeetCode - Restore IP Addresses

class Solution {
public:
    vector<string> restoreIpAddresses(string s) {
        vector<string> res;
        helper(s, "", 4, res);
        return res;
    }
    void helper(string s, string out, int n, vector<string>& res){
        if(n==0 && !s.empty()) return;
        else if(n==0 && s.empty()){
            res.push_back(out);
        }else if(n==1){
            if(isvalid(s, s.size())) helper("", out+s, n-1, res);
        }else{
            for(int i=1;i<4;i++){
                if(i<=s.size()){
                    string tt = s.substr(0,i);
                    if(!isvalid(tt, i)) continue;
                    helper(s.substr(i), out+tt+'.', n-1, res);
                }
            }
        }
    }
    bool isvalid(string s, int i){
        int t = atoi(s.c_str());
        if(t>255 || t<0 || to_string(t).size()!=i) return false;
        return true;
    }
};

Notes

1. k != std::to_string(val).size()

2.atoi(s.c_str()) is better than stoi()<--cannot handle ""