leetcode---Restore IP Addresses---回溯

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 ok(string sub)
    {
        if(sub.length() == 1)
            if("0" <= sub && sub <= "9")
                return true;
        if(sub.length() == 2)
            if("10" <= sub && sub <= "99")
                return true;
        if(sub.length() == 3)
            if("100" <= sub && sub <= "255")
                return true;
        return false;
    }

    void dfs(string s, int dep, int start, vector<string> &ans, vector<string> &tmp)
    {
        if(start == s.length())
        {
            if(dep == 4)
            {
                string ip = tmp[0];
                for(int i=1; i<4; i++)
                    ip += "." + tmp[i];
                ans.push_back(ip);
            }
            return;
        }
        for(int k=1; k<=3 && start+k-1string subIP = s.substr(start, k);
            if(ok(subIP))
            {
                tmp.push_back(subIP);
                dfs(s, dep+1, start+k, ans, tmp);
                tmp.pop_back();
            }
        }
    }

    vector<string> restoreIpAddresses(string s) 
    {
        vector<string> ans;
        vector<string> tmp;
        if(s.length() <4 || s.length() > 12)
            return ans;
        dfs(s, 0, 0, ans, tmp);
        return ans;
    }
};