LeetCode OJ - 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)

分析: DFS

class Solution {
private:
    vector<string> ret;
public:
    bool check(string &s, int beg, int end)
    {
        string ip = s.substr(beg, end - beg + 1);
        if (ip.size() == 1)
            return "0" <= ip && ip <= "9";
        else if (ip.size() == 2)
            return "10" <= ip && ip <= "99";
        else if (ip.size() == 3)
            return "100" <= ip && ip <= "255";
        else
            return false;
    }
    
    void dfs(string &s, int start, int level, vector<string> &item)
    {
        if(level == 4) {
            if(start == s.size()) {
                string s;
                for(int i = 0; i < 4; i++) {
                    s += item[i];
                    if(i != 3) s += ".";
                }
                ret.push_back(s);
            }
            return ;
        }
        
        for(int i = start; i < s.size(); i++) {
            if(check(s, start, i)) {
                item.push_back(s.substr(start, i - start + 1));
                dfs(s, i + 1, level + 1,item);
                item.pop_back();
            }
        }
    }
    
    vector<string> restoreIpAddresses(string s) {
        vector<string> item;
        dfs(s, 0, 0, item);
        return ret;
    }
};