Do you allow linear space?
But there is a catch that is very likely not obvious here. All the numbers in the array are (were) unique (and the zero was not present).
Some information is actually missing/not obvious and interviewers may actually evaluate your reaction to this situation because this would also model (to some degree) a real life situation.
So I added the missing bits (and fixed typos).
You have an array size of n of integers from 1 to n, that is not sorted. There are no duplicate numbers in this array.
One of the numbers got changed to 0. Given the array after the change return the changed number in linear time and constant space.
Assume the same array but two of the numbers got changed to 0. Given an array after the change return the changed numbers in linear time and constant space.
The first part of this problem should be simple. The second part is more tricky.