lintcode

class Solution {
public:
    /*
     * @param s: the IP string
     * @return: All possible valid IP addresses
     */
    vector<string> restoreIpAddresses(string &s) {
        // write your code here
        vector<string> ans;
        dfs(s, 0, ans, "", 0);
        return ans;
    }
    void dfs(string &s, int pos, vector<string> &ans, string res, int cnt){
        if(pos >= s.length() && cnt == 4){
                ans.push_back(res);
                return ;
        } else {
                for(int i = 0; i <= 2; ++i){
                        if(pos + i < s.length()){
                                string tmp = s.substr(pos, i + 1);
                                int num = atoi(tmp.c_str());
                                if((num == 0 && tmp.length() != 1) || (num != 0 && tmp[0] == '0')) continue;
                                string nres = "";
                                if(i == 2){
                                       if(num < 256){
                                        if(pos != 0)
                                                nres = res + "." + tmp;
                                        else    nres = res + tmp;
                                        dfs(s, pos + i  + 1, ans, nres, cnt + 1);
                                       }
                                } else {
                                        if(pos != 0)
                                                nres = res + "." + tmp;
                                                else nres = res + tmp;
                                        dfs(s, pos + i + 1, ans, nres, cnt + 1);
                                }
                        }
                }
        }
    }
};

ip地址的每个区域值不大于255 且不能有前导零