320. Generalized Abbreviation

class Solution {
public:
    vector<string> generateAbbreviations(string word) {
        vector<string> res;
        for (int i = 0; i < (1 << word.size()); i++)
            res.push_back(buildAbbr(word, i));
        return res;
    }
private:
    string buildAbbr(string &word, int k) {
        string res = "";
        int count = 0;
        for (int i = word.size() - 1; i >= 0 ; i--) {
            if ((k & 1) == 0) count++;  //aaaaaa if (k & 1 == 0) is wrong!!!!
            else {
                if (count) res = to_string(count) + res;
                res = word[i] + res;
                count = 0;
            }
            k >>= 1;
        }
        if (count) res = to_string(count) + res;
        return res;
    }
};