Counting Bits

vector<int> countBits(int num) {
        vector<int> res;
        res.push_back(0);
        if(num==0)return res;
        res.push_back(1);
        for(int i=2;i<=num;i++)
        {
            int a=res[i/2]+res[i%2];
            res.push_back(a);
        }
        return res;
    }