LeetCode 第 338 题 (Counting Bits)

Given a non negative integer number num. For every numbers i in the range 0 ≤ i ≤ num calculate the number of 1’s in their binary representation and return them as an array.

Example:

For num = 5 you should return [0,1,1,2,1,2].

Follow up:

It is very easy to come up with a solution with run time
O(n*sizeof(integer)). But can you do it in linear time O(n) /possibly in
a single pass?

Space complexity should be O(n).

Can you do it like a boss? Do it without using any builtin function like __builtin_popcount in c++ or in any other language.

class Solution {
public:
vector<int> countBits(int num) {
vector<int> ret;
ret.push_back(0);
for(int i = 1; i <= num; i++)
{
int bits = ret[i >> 1] + (i & 1);
ret.push_back(bits);
}
return ret;
}
};