[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 {

private:

    vector<string> ret;

    int pos[4];

public:

    bool check(string &s, int beg, int end)

    {

        string ip(s, beg, end - beg + 1);

        if (ip.size() == 1)

            return "0" <= ip && ip <= "9";

        else if (ip.size() == 2)

            return "10" <= ip && ip <= "99";

        else if (ip.size() == 3)

            return "100" <= ip && ip <= "255";

        else

            return false;

    }

    

    void dfs(int dep, int maxDep, string &s, int start)

    {

        if (dep == maxDep)

        {

            if (start == s.size())

            {

                int beg = 0;

                string addr;

                for(int i = 0; i < maxDep; i++)

                {

                    string ip(s, beg, pos[i] - beg + 1);

                    beg = pos[i] + 1;

                    addr += i == 0 ? ip : "." + ip;

                }

                ret.push_back(addr);    

            }

            return;

        }

        

        for(int i = start; i < s.size(); i++)

            if (check(s, start, i))

            {

                pos[dep] = i;

                dfs(dep + 1, maxDep, s, i + 1);               

            }

    }

    

    vector<string> restoreIpAddresses(string s) {

        // Start typing your C/C++ solution below

        // DO NOT write int main() function

        ret.clear();

        dfs(0, 4, s, 0);

        return ret;

    }

};