I'm studying Computer science and this has confused me for a long time since our professor didn't give any proof.
When changing from 2's complement to the positive value, we can go in reverse (by subtracting 1, then using 1's complement), and that's clear why it works.
But our professor told us another method which is taking the number, using 1's complement, THEN adding 1.
I don't understand why the second method works.