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 {
    int MIN(int a, int b)
	{
		if (a > b)
			return b;

		return a;
	}

    int str2int(string &str) {
        
        int size = str.size();
        int retVal = 0;
        
        for (int i = 0; i < size; i++)
        {
            retVal = retVal*10+(str[i]-'0');
        }
        
        return retVal;
    }
    
    bool isValidNum(int num)
    {
        if (num > 255)
            return false;
        
        return true;
    }

    void foo(string remainString, int remainNum, string &curString, vector<string> &retStrings)
    {        
        if  ((remainNum == 0) && (remainString.size() == 0))
        {
            retStrings.push_back(curString);
            return;
        }
        
		if ((remainString.size() > remainNum*3) || (remainString.size() < remainNum))
            return;
        
        if (remainNum > 0 && remainNum < 4)
            curString += ".";

		int maxTryNum = MIN(3, remainString.size());
        for (int tryNum = maxTryNum; tryNum >= 1; tryNum--)
        {
            string tryString = remainString.substr(0, tryNum);
            
            if (tryNum > 1 && tryString[0] == '0')
                continue;
            
            if (tryNum == 3 && !isValidNum(str2int(tryString)))
                continue;
            
            curString += tryString;
            foo(remainString.substr(tryNum, remainString.size()-tryNum), remainNum-1, curString, retStrings);
            curString = curString.substr(0, curString.size()-tryNum);
        }
        
        if (remainNum > 0 && remainNum < 4)
    		curString = curString.substr(0, curString.size()-1);
    }
    
public:    
    vector<string> restoreIpAddresses(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        
        vector<string> retStrings;
        string curString;
        
        foo(s, 4, curString, retStrings);
        
        return retStrings;
        
        
    }
};