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)

貌似刚csdn抽风了， 回溯法， 找四个符合条件的数字组成一个ip， 注意三个问题：

1. 符合条件的数字： 1-3位， 0-255之间，且不能010出现

2. 停止条件， 有四个数字，且无剩余数字

3. 回溯， push, next, pop...

class Solution {
public:
    vector<string> restoreIpAddresses(string s) {
    	vector<string> res;
    	vector<string> solution;
    	scanIp(s,0,res,solution);
    	return res;
    }

private:
    bool isValid(const string& s){
    	int n=s.size();
    	if (n==0 || n>3)
    		return false;
    	if (s[0]=='0' && n>1)
    		return false;
    	if (n==3 && atoi(s.c_str())>255)
    		return false;
    	return true;
    }

    void scanIp(const string& s, int index, vector<string>& res, vector<string>& solution){
    	if (solution.size()==4){ // stopping condition, have 4 numbers
    		if (index==s.size()){ // there is not left numbers, otherwise, invalid
    			string tmp=solution[0];
    			for (int i=1; i<4; i++)
    				tmp=tmp+"."+solution[i];
    			res.push_back(tmp);
    		}
    		return;
    	}

    	string num;
    	for (int i=index; i<s.size()&& i<index+3; i++){
    		num.push_back(s[i]);
    		if (isValid(num)){
    			solution.push_back(num);
    			scanIp(s,i+1, res, solution);
    			solution.pop_back();
    		}
    	}
    }
};