1 class Solution { 2 public: 3     vector
     
       restoreIpAddresses(string s) { 4         vector
      
        ret; 5         if(s.size() > 12) 6             return ret; 7         for(int i = 0; i < s.size(); i ++) 8         {
   // [0, i] 9             string ip1 = s.substr(0, i+1);10             if ( !check(ip1) ) break;11             for(int j = i+1; j < s.size(); j ++)12             {
   // [i+1, j]13                 string ip2 = s.substr(i+1, j-i);14                 if ( !check(ip2) ) break;15                 for(int k = j+1; k < s.size()-1; k ++)16                 {
   // [j+1, k], [k+1, s.size()-1]17                     string ip3 = s.substr(j+1, k-j);18                     string ip4 = s.substr(k+1);19                     if(check(ip3) && check(ip4))20                     {21                         string ip = ip1 + "." + ip2 + "." + ip3 + "." + ip4;22                         ret.push_back(ip);23                     }24                 }25             }26         }27         return ret;28     }29     bool check(string ip)30     {31         int value = stoi(ip);32         if(ip[0] == '0')    return ip.size() == 1;33         return value <= 255 ? true : false;34     }35 };