LeetCode: Counting Bits

class Solution {
public:
    vector<int> countBits(int n) {
        vector <int> ans;
        ans.push_back(0);
        if(n == 0) return ans;
        ans.push_back(1);
        if(n == 1) return ans;

        for(int i=2;i<=n;i++)
            ans.push_back(ans[i >> 1] + (i & 1  == 1));

        return ans;
    }
};