Valid Ip Addresses

• Time : $O(3^4)$
• Space : $O(1)$

void helper(string s, int p, vector<string>& res, string now, int cut = 0) {
    if(cut == 3) {
        if(p == s.length()) return;
        if(p + 3 < s.length()) return;
        if(s[p] == '0' and p + 1 != s.length()) return;
        int part = stoi(s.substr(p));
        if(0 <= part and part <= 255) {
            now += s.substr(p);
            res.push_back(now);
        }
        return;
    }
    if(s[p] == '0') helper(s,p+1,res,now + "0.", cut + 1);
    else {
        int part = 0;
        for(int i = p; i < min(p + 3,(int)s.length()); i++) {
            now.push_back(s[i]);
            part = part * 10 + s[i] - '0';
            if(0 <= part and part <= 255)
                helper(s,i+1,res,now + ".", cut + 1);
        }
    }
}
vector<string> Solution::restoreIpAddresses(string A) {
    vector<string> res;
    helper(A,0,res,"");
    sort(begin(res),end(res));
    return res;
}