* Leetcode 93. Restore IP Addresses

https://leetcode.com/problems/restore-ip-addresses/description/

简单dfs，结合具体ip的格式要求剪枝即可

class Solution {public:    vector<string> restoreIpAddresses(string s) {        vector<string> ans;        string prefix = "";        dfs(s, 0, 0, prefix, ans);        return ans;    }        bool legal(string s) {        if ( s[0] == '0' && s.size() > 1 ) return false;                return ( stoi(s) <=255 );    }        void dfs(string ip, int cur, int already_cnt, string prefix, vector<string> &ans) {        if (cur >= ip.size()) {            return ;        }        for (int i = 0; i < 3; i++) {            if (cur + i >= ip.size() )continue;            string seg = ip.substr(cur, i+1);            if (legal(seg)) {                if (already_cnt == 3) {                    string one_ans = prefix + seg;                    if (one_ans.size() == ip.size() + 3)                        ans.push_back(prefix + seg);                } else {                    dfs(ip, cur + i + 1, already_cnt + 1, prefix + seg + '.', ans);                }            }        }    }    };