[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)

DFS!

 1 class Solution {
 2 public:
 3     bool isVail(string s) {
 4         if (s.length() != 1 && s[0] == '0') 
 5             return false;
 6         int n = s[0] - '0';
 7         for (int i = 1; i < s.size(); ++i) {
 8             n *= 10;
 9             n += (s[i] - '0');
10         }
11         return n <= 255;
12     }
13     
14     void findNext(vector<string> &res, string &s, string substr, int cnt, int idx) {
15         if (cnt < 4 && cnt > 0) 
16             substr.push_back('.');
17         if (cnt == 4 && idx == s.length()) {
18             res.push_back(substr);
19             return;
20         } else if (cnt == 4 && idx != s.length()) {
21             return;
22         }
23         
24         string digit;
25         for (int len = 1; len <= 3 && idx + len <= s.length(); ++len) {
26             digit = s.substr(idx, len);
27             if (isVail(digit)) {
28                 findNext(res, s, substr + digit, cnt + 1, idx + len);
29             }
30         }
31     }
32     
33     vector<string> restoreIpAddresses(string s) {
34         vector<string> res;
35         findNext(res, s, "", 0, 0);
36         return res;
37     }
38 };