LeetCode - Refresh - Restore IP Addresses

class Solution {
public:
    bool isIp(string s) {
        int len = s.size(), ip = 0;
        if (len == 0 || len > 4 || len > 1 && s[0] == '0') return false;
        for (int i = 0; i < len; i++) {
            ip = ip*10 + int(s[i] - '0');
        }
        if (ip > 255) return false;
        return true;
    }
    
    vector<string> restoreIpAddresses(string s) {
        vector<string> result;
        int len = s.size();
        string ip1, ip2, ip3, ip4;
        if (len < 4 || len > 12) return result;
        for (int i = 0; i < 3; i++) {
            if (i+1 > len) continue;
            ip1 = s.substr(0, i+1);
            if (isIp(ip1)) {
                for (int j = 0; j < 3; j++) {
                    if (i+j+2 > len) continue;
                    ip2 = s.substr(i+1, j+1);
                    if (isIp(ip2)) {
                        for (int k = 0; k < 3; k++) {
                            if (i+j+k+3 > len) continue;
                            ip3 = s.substr(i+j+2, k+1);
                            ip4 = s.substr(i+j+k+3);
                            if (isIp(ip3) && isIp(ip4)) {
                                result.push_back(ip1 + '.' + ip2 + '.' + ip3 + '.' + ip4);
                            }
                        }   
                    }
                }
            }
        }
        return result;
    }
};