Programming question: Given a number, eg. 8, try to calculate and output a list/array of

that in each number's binary form - how many '1' in each number? (should not use string. count('1') in the Python. Efficiency is most important!)

Example: number is 8.

Expected output is - [0, 1, 1, 2, 1, 2, 2, 3, 1]

See some good code snippet (listed below), but not quite understand it? Can you help explain?

```code:

from typing import List

def countBits(num: int) -> List[int]:

""" count all numbers: from 0 to num (eg. 8)

-each number's binary bit in '1':

>>> countBits(8)

[0, 1, 1, 2, 1, 2, 2, 3, 1]

"""

res = [0]

while len(res) <= num:

for i in res[:num+1 - len(res)]: # :8 - 7- 6 -5 1

res += [i + 1]

return res