leetcode[93]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 {
public:
bool valid(string num)
{
    if(num.size()==1)return (num>="0"&&num<="9");
    if(num.size()==2)return (num>="10"&&num<="99");
    if(num.size()==3)return (num>="100"&&num<="255");
    return false;
}
void find(vector<string> &res, vector<string> &ips, int kth, int begin,string &s)
{
    if (begin==s.size())
    {
        if(kth==5)
        {
            string tmp=ips[0];
            for (int i=1;i<ips.size();i++)
            {
                tmp+=('.'+ips[i]);
            }
            res.push_back(tmp);
            return;
        }
        return;
    }
    for (int k=1;k<4&&(begin+k-1<s.size());k++)
    {
        string ip=s.substr(begin,k);
        if (valid(ip))
        {
            ips.push_back(ip);
            find(res,ips,kth+1,begin+k,s);
            ips.pop_back();
        }
    }
    return;
}
vector<string> restoreIpAddresses(string s)
{
    vector<string> res;
    if(s.size()<4||s.size()>12)return res;
    vector<string> ips;
    find(res, ips, 1, 0,s);
    return res;
}
};