Count number of 1's in binary representation

Question

Efficient way to count number of 1s in the binary representation of a number in O(1) if you have enough memory to play with. This is an interview question I found on an onli

Answer 1

There's only one way I can think of to accomplish this task in O(1)... that is to 'cheat' and use a physical device (with linear or even parallel programming I think the limit is O(log(k)) where k represents the number of bytes of the number).

However you could very easily imagine a physical device that connects each bit an to output line with a 0/1 voltage. Then you could just electronically read of the total voltage on a 'summation' line in O(1). It would be quite easy to make this basic idea more elegant with some basic circuit elements to produce the output in whatever form you want (e.g. a binary encoded output), but the essential idea is the same and the electronic circuit would produce the correct output state in fixed time.

I imagine there are also possible quantum computing possibilities, but if we're allowed to do that, I would think a simple electronic circuit is the easier solution.