lintcode - 恢复ip地址

 class Solution {
 public:
     /*
      * @param s: the IP string
      * @return: All possible valid IP addresses
      */
     vector<string> restoreIpAddresses(string &s) {
         // write your code here
         vector<string> ans;
         dfs(s, , ans, "", );
         return ans;
     }
     void dfs(string &s, int pos, vector<string> &ans, string res, int cnt){
         if(pos >= s.length() && cnt == ){
                 ans.push_back(res);
                 return ;
         } else {
                 for(int i = ; i <= ; ++i){
                         if(pos + i < s.length()){
                                 string tmp = s.substr(pos, i + );\
                                 int num = atoi(tmp.c_str());
                                 if((num ==  && tmp.length() != ) || (num !=  && tmp[] == '')) continue;
                                 string nres = "";
                                 if(i == ){
                                        if(num < ){
                                         if(pos != )
                                                 nres = res + "." + tmp;
                                         else    nres = res + tmp;
                                         dfs(s, pos + i  + , ans, nres, cnt + );
                                        }
                                 } else {
                                         if(pos != )
                                                 nres = res + "." + tmp;
                                                 else nres = res + tmp;
                                         dfs(s, pos + i + , ans, nres, cnt + );
                                 }
                         }
                 }
         }
     }
 };

ip地址的每个区域值不大于255 且不能有前导零