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)

C++实现代码：

#include<iostream>

#include<string>

#include<vector>

#include<cstdlib>

using namespace std;



class Solution {

public:

    vector<string> restoreIpAddresses(string s) {

        vector<string> ret;

        if(s.empty())

            return ret;

        string str;

        helper(s,0,4,ret,str);

        return ret;

    }

    void helper(string &s,int start,int num,vector<string> &ret,string &str)

    {

        if(num==0)

        {

            //删除最后一个.

            str.pop_back();

            if(str.size()==s.size()+3)

            {

                ret.push_back(str);

            }

            return;

        }

        int len=str.length();

        int i;

        //start+i<=(int)s.length()是为了防止最后一个不能达到3个字符而取到空字符

        //i用来限制子串的长度，最多为3个字符

        for(i=1;i<=3&&start+i<=(int)s.length();i++)

        {

            string tmp=s.substr(start,i);

            if(!isValid(tmp))

                continue;

            str+=tmp+'.';

            helper(s,start+i,num-1,ret,str);

            str.erase(len,i+1);

        }

    }

    bool isValid(string s)

    {

        if(s.empty()||s.size()>3)

            return false;

        if(s[0]=='0'&&s.length()>1)

            return false;

        int num=atoi(s.c_str());

        if(num>=0&&num<=255)

            return true;

        return false;

    }

};



int main()

{

    Solution s;

    vector<string> result=s.restoreIpAddresses(string("25525511135"));

    for(auto a:result)

        cout<<a<<endl;

}