1 class Solution { 2 public: 3     vector<string> restoreIpAddresses(string s) { 4         vector<string> res; 5         string current; 6         if (s.size() > 12 || s.size() < 4) return res; 7         else { 8             checkIp(s, res, current, 0, 1); 9             return res;10         }11     }12 private:13     void checkIp(string s, vector<string>& res, string& current, int startPos, int step)14     {15         if (startPos > s.size() - 1) return;16         if (step == 4) {17             if (startPos < s.size() - 3) return;18             else {19                 if (isValid(s, startPos, s.size() - 1)) {20                     current.append(s, startPos, s.size() - startPos);21                     res.push_back(current);22                     current.erase(current.size() + startPos - s.size(), s.size() - startPos);23                     return;24                 }25             }26         }27         else {28             for (int i = 0; i < 3 && startPos + i < s.size(); ++i) {29                 if (isValid(s, startPos, startPos + i)) {30                     current.append(s, startPos, i + 1);31                     current.push_back(‘.‘);32                     checkIp(s, res, current, startPos + i + 1, step + 1);33                     current.pop_back();34                     current.erase(current.size() - i - 1, i + 1);35                 }36             }37         }38     }39 40     bool isValid(string s, int left, int right) {41         if (s[left] == ‘0‘ && right > left) return false; // 这里需要判断如果以0开头，则只能有一位042         int num = 0;43         for (int i = left; i <= right; ++i) {44             num *= 10;45             num += s[i] - ‘0‘;46         }47         if (num >= 0 && num < 256) return true;48         else return false;49     }50 };