time O(n), space O(1)
time O(n), space O(1)

- count the bit for each position.
- sum of (C(n-pos[k],1) * C(pos[k],1)) for each position.

Signed-off-by: hwchiu <5d5bf96f0f7137bc5ec8b0b405b00e5a90f259c2@cs.nctu.edu.tw>

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