Number is Fancy

// my solution for: http://www.geeksforgeeks.org/check-if-a-given-number-is-fancy/

// Precondition: i only contains digit chars
bool is_fancy(const string& i)
{
    static const map<char, char> toFancy{
      {'0', '0'}, {'1', '1'}, 
      {'6', '9'}, {'8', '8'}, {'9', '6'},
      {'2', 0}, {'3', 0}, {'4', 0}, {'5', 0}, {'7', 0},
    };
    
    auto fw = i.begin(); // forward
    auto bw = i.rbegin(); // backward
    
    while ((fw != end(i)) && (toFancy.at(*bw++) == *fw++));
    return fw == end(i);
}