Leetcode OJ : Restore IP Addresses backtrack暴搜 C++ solution

class Solution {
    public:
    vector<string> ret;
    string src;
    int len;
    unordered_set<string> added;
    vector<string> restoreIpAddresses(string s) {
        if ( s.empty() )  return ret;
        src = s;
        len = src.size();
        backTrack(0,0,  "");
        return ret;
    }

    void backTrack(int pos, int dot_cnt, string substr) {
        if ( dot_cnt > 4 ) return;
        if ( pos >= len && dot_cnt == 4 ) {
            substr.pop_back();
            if ( added.find(substr) == added.end() ) {
                ret.push_back( substr );
                added.insert( substr );
            }
            return;
        }
        char buffer[16];
        int tx = len - pos > 3 ? 3 : len - pos;
        for ( int i = 1; i <= tx; i++ ) {
            string temp = src.substr( pos, i );
            int number = atoi( temp.c_str() );
            sprintf( buffer, "%d", number );
            string str(buffer);
            if ( str != temp ) continue;
            string newsub = substr + temp + ".";
            if ( number >= 0 && number <= 255 ) {
                backTrack( pos + i, dot_cnt + 1,  newsub );
            }
        }
    }
};