c++ RLE encoding

rle_encode.cpp

string compression(const string & str){
    int i = str.size();
    string letters;

    for (int j = 0; j < i; ++j){
        int count = 1;
        while (str[j] == str[j+1]){
            count++;
            j++;
        }
        letters += std::to_string(count);
        letters.push_back(str[j]);
    }
    return letters;
}