[LeetCode] 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)

class Solution {
public:
    vector<string> restoreIpAddresses(string s) {
        vector<string> res;
        int len = s.size();
        if(len<4)
            return res;
        for(int i=0;i<len-2;i++)
            for(int j=i+1;j<len-1;j++)
                for(int k=j+1;k<len;k++){
                    if(i-0<=3 && j-i<=3 && k-j<=3){
                        
                     string sRestore =IsValidAndRestore(i,j,k,s);
                     if(sRestore != "" && find(res.begin(),res.end(),sRestore)==res.end())
                       res.push_back(sRestore);
                    }
                
                
                }//end for
    return res;
    }
private:
    
    bool valid(string s){//各子串是否合法
       istringstream is(s);
       int res;
       is>>res;
       
       if((s.size()>1 && s[0]=='0')|| res>255 || res<0)
           return false;
       else
           return true;
       
    }//end valid
    string IsValidAndRestore(int x,int y,int z,string s){
        string s1 = s.substr(0,x);
        string s2 = s.substr(x,y-x);
        string s3 = s.substr(y,z-y);
        string s4 = s.substr(z);
        string Res;
        if(valid(s1) && valid(s2) && valid(s3) && valid(s4)){
           return s1+"."+s2+"."+s3+"."+s4;
        }
        return Res;
    }//end IsValid
};