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:
    std::vector<std::string> restoreIpAddresses(std::string s) {
        std::vector<std::string> res;
		if(s.size() < 4 || s.size() > 12) return res;
		dfs(s,1,"",res);
#if 1
		for (int i = 0; i < res.size(); i++)
		{
			std::cout << res[i] << std::endl;
		}
#endif // 1

		return res;
    }
private:
	void dfs(std::string s,int n, std::string ip,std::vector<std::string> &res)
	{
		if(s.size() == 0) return;
		if(n == 4 && isValid(s))
		{
			ip += s;
			res.push_back(ip);
			return;
		}
		for (int i = 1; i <= 3; i++)
		{
			if(i > s.size()) break;
			std::string t = s.substr(0,i);
			if(isValid(t))
			{
				std::string tmp = s.substr(i);
				dfs(tmp,n+1,ip+t+".",res);
			}
		}
	}
	bool isValid(std::string s)
	{
		if(s.size() == 0 || (s.size() > 1 && s[0] == '0')) return false;
		int n = std::atoi(s.c_str());
		if(0 <= n && n <= 255) return true;
		return false;
	}
};

leetcode - Restore IP Addresses