Restore IP Addresses

Leetcode问题： 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"].

思路：递归尝试。ip地址十进制形式的一个段的范围是0~255，而且注意避免0*和0**的情况。当字符串到头或者到达4个段的时候，停止尝试。

代码：

class Solution {
public:
    vector<string> restoreIpAddresses(string s) {
        vector<string> result;
        if(s.size() == 0)
            return result;
        string cur = "";
        find(s, 0, 0, cur, result);
        return result;
    }
    
    void find(string s, int from, int count, string cur, vector<string> &result)
    {
        if(from == s.size() && count == 4)
        {
            cur.erase(cur.end()-1);
            result.push_back(cur);
            return;
        }
        if(count == 4)
            return;
        if(from == s.size())
            return;
        
        
        if(from+2 < s.size() && s[from] != '0' && s.substr(from, 3) <= "255")
        {
            cur += s.substr(from, 3) + ".";
            find(s, from+3, count+1, cur, result);
            cur.erase(cur.end()-4, cur.end());
        }
        if(from+1 < s.size() && s[from] != '0')
        {
            cur += s.substr(from, 2) + ".";
            find(s, from+2, count+1, cur, result);
            cur.erase(cur.end()-3, cur.end());
        }
        
            cur += s.substr(from, 1) + ".";
            find(s, from+1, count+1, cur, result);
    }
};