#include<iostream>#include<string>#include<vector>#include<cstdlib>using namespace std;class Solution {public:    vector<string> restoreIpAddresses(string s) {        vector<string> ret;        if(s.empty())            return ret;        string str;        helper(s,0,4,ret,str);        return ret;    }    void helper(string &s,int start,int num,vector<string> &ret,string &str)    {        if(num==0)        {            //删除最后一个.            str.pop_back();            if(str.size()==s.size()+3)            {                ret.push_back(str);            }            return;        }        int len=str.length();        int i;        //start+i<=(int)s.length()是为了防止最后一个不能达到3个字符而取到空字符        //i用来限制子串的长度，最多为3个字符        for(i=1;i<=3&&start+i<=(int)s.length();i++)        {            string tmp=s.substr(start,i);            if(!isValid(tmp))                continue;            str+=tmp+‘.‘;            helper(s,start+i,num-1,ret,str);            str.erase(len,i+1);        }    }    bool isValid(string s)    {        if(s.empty()||s.size()>3)            return false;        if(s[0]==‘0‘&&s.length()>1)            return false;        int num=atoi(s.c_str());        if(num>=0&&num<=255)            return true;        return false;    }};int main(){    Solution s;    vector<string> result=s.restoreIpAddresses(string("25525511135"));    for(auto a:result)        cout<<a<<endl;}