leetcode - Restore IP Addresses

class Solution {
public:
bool isValid(string &s, int start, int end){
int len = end-start+1;
if (len<=0 || len>3)
return false;
if (len>1 && s.at(start)=='0')
return false;
int _val  = atoi(s.substr(start,len).c_str());
if (_val>=0 && _val<=255)
return true;
return false;
}
vector<string> restoreTwo(string & s, int start, int end){
int len = end-start+1;
vector<string> rlt;
if (len<=0 || len>6)
return rlt;
vector <string> rlts;
for (int i=start;i<end; i++){
if (isValid(s,start, i)&& isValid(s,i+1,end))
rlts.push_back(s.substr(start, i-start+1)+"."+s.substr(i+1, end-i));
}
return rlts;
}
vector<string> restoreIpAddresses(string s) {
// IMPORTANT: Please reset any member data you declared, as
// the same Solution instance will be reused for each test case.
int len = s.length();
vector<string> rlts;
for (int i=0; i<len-1; i++){
if (i+1<2 || len-i-1<2 || i+1>6 || len-i-1>6)
continue;
vector<string> left = restoreTwo(s,0,i);
vector<string> right = restoreTwo(s,i+1,len-1);
int m = left.size();
int n = right.size();
if (m<=0 || n<=0)
continue;
for (int p=0; p<m; p++)
for (int q=0; q<n; q++){
rlts.push_back(left[p]+"."+right[q]);
//cout << "[" << left[p]+"."+right[q] << "]" << endl;
}
}
return rlts;
}
};