第九周：[Leetcode]93. 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)

---

使用三个变量记录前三段地址的长度（前三段地址长度确定了第四段自然也确定），枚举每种长度的地址数值，看是否合法，均合法的话就加入返回值中。

---

class Solution {public:    bool valid(string s){    if(s.length() > 1 && s[0] == '0')        return false;    int n = 0;    for(int i = 0;i < s.length();i++){        n *= 10;        n += s[i] - '0';    }    return n <= 255;}vector<string> restoreIpAddresses(string s) {    vector<string> ret;    for(int first = 1;first <= 3;first ++)        for(int second = 1;second + first <= s.length() - 2 && second <= 3;second ++)            for(int third = 1;third + second + first <= s.length() - 1 && third <= 3;third ++){                int fourth = s.length() - third - second - first;                if(fourth > 0 && fourth < 4){                    string s1 = s.substr(0,first);                    string s2 = s.substr(first,second);                    string s3 = s.substr(first + second,third);                    string s4 = s.substr(first + second + third,fourth);                    if(valid(s1) && valid(s2) && valid(s3) && valid(s4)){                        ret.push_back(s1 +'.'+ s2 +'.'+ s3 +'.'+ s4);                    }                }            }    return ret;}};