[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)

class Solution {public:    vector<string> restoreIpAddresses(string s) {        vector<string> res;        string ip;        dfs(s,0,0,ip,res);        return res;            }    void dfs(string &s, size_t start, size_t step, string ip, vector<string> &res) {        if (start == s.size() && step == 4) {            ip.resize(ip.size()-1);            res.push_back(ip);            return ;        }        if (s.size()-start > (4-step)*3) {            return ;        }        if (s.size()-start < (4-step)) {            return ;        }        int num = 0;        for (int i = start; i < start + 3; i++) {            num = num*10 + (s[i]-'0');            if (num <= 255) {                ip += s[i];                dfs(s, i+1, step+1, ip+'.', res);            }            if (num == 0) {                return ;            }        }    }};