Restore IP Addresses

Given a string containing only digits, restore it by returning all possible valid IP address combinations.

For example:
Given "25525511135",

return ["255.255.11.135", "255.255.111.35"]. (Order does not matter)

 1 class Solution { 2 public: 3     vector<string> restoreIpAddresses(string s) { 4         5         vector<string> result; 6         string tmp=""; 7         bt(s,result); 8         return result; 9        10     }11    12     void bt(string s,vector<string> &result,int start=0,string tmp="",int left=5)13     {14         left--;15         int n=s.length();16        17         //找到的条件18         if(left==0&&start==n)19         {20             //去除最后一个点21             tmp.pop_back();22             result.push_back(tmp);23             return;24         }25        26         //停止条件27         if(left==0||(n-start)/left>3||(n-start)/left==0||start>n)28         {29             return;30         }31  32        33         string tmpStr;34         if(n-start>=1)35         {36             tmpStr=tmp;37             string str1=s.substr(start,1);38             tmp+=str1;39             bt(s,result,start+1,tmp+".",left);40             tmp=tmpStr;41         }42        43         if(n-start>=2&&s[start]!=‘0‘)44         {45             tmpStr=tmp;46             string str2=s.substr(start,2);47             tmp+=str2;48             bt(s,result,start+2,tmp+".",left);49             tmp=tmpStr;50         }51        52         if(n-start>=3&&s[start]!=‘0‘)53         {54             tmpStr=tmp;55             string str3=s.substr(start,3);56             int n = atoi(str3.c_str());57            58             if(n<=255)59             {60                 tmp+=str3;61                 bt(s,result,start+3,tmp+".",left);62                 tmp=tmpStr;63             }64         }65        66     }67 };