class Solution {public:    vector<string> restoreIpAddresses(string s) {        vector<string> ips;        vector<int> ip;        dfs(s, 0, ip, ips);        return ips;    }        void dfs(string& s, int pos, vector<int>& ip, vector<string>& ips) {        int len = s.length();        int fid = ip.size();        if (len == pos) {            if (fid == 4) {                ips.push_back(intip2str(ip));            }            return;        }                if (3 * (4 - fid) < len - pos) return;　　// avoid TL                int cur = 0;        for (int i=pos; i<len && i < pos+3; i++) {            if (i != pos && cur == 0) break; 　　// no leading zero in ip addr            cur = cur * 10 + s[i] - ‘0‘;            if (cur > 255) break;            ip.push_back(cur);            dfs(s, i + 1, ip, ips);            ip.pop_back();        }    }        string intip2str(vector<int> &ip) {        if (ip.size() != 4 ) {            return "format error. number of fields should be 4";        }        string strip;        for (int i=0; i<4; i++) {            strip.append(to_string(ip[i]));            strip.append(i==3 ? "" : ".");        }        return strip;    } };