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:
    bool  check(string s){
        if(atoi(s.c_str()) > 255) return false;
        if(s.size() >1 && s[0] == '0')return false;
        return true;
    }
    void dfs(string s, vector<string> &cur){
        if(cur.size() == 3 ){
            if(s.size() >=1 &&check(s)){
                cur.push_back(s);
                res.push_back(cur[0]+'.'+cur[1]+'.'+cur[2]+'.'+cur[3]);
                cur.pop_back();
            }
            return ;
        }
        
        for(int i = 1; i <= 3 && i <= s.size(); i++){
             string temp = s.substr(0,i) ;
             if(check(temp)){
                cur.push_back(temp);
                dfs(s.substr(i,s.size()-i),cur);
                cur.pop_back();
             }
        }
    
    }
    vector<string> restoreIpAddresses(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        res.clear();
        if(s.size()>12 || s.size() <4) return res;
        vector<string> cur;
        dfs(s,cur);
        return res;
    }
private:
    vector<string> res;
};