Finding repeating signed integers with O(n) in time and O(1) in space

Question

(This is a generalization of: Finding duplicates in O(n) time and O(1) space)

Problem: Write a C++ or C function with time and space complexities of O(n) and O(1) respec

Answer 1

There is a tricky problem with definitions here. What does O(n) mean?

Konstantin's answer claims that the radix sort time complexity is O(n). In fact it is O(n log M), where the base of the logarithm is the radix chosen, and M is the range of values that the array elements can have. So, for instance, a binary radix sort of 32-bit integers will have log M = 32.

So this is still, in a sense, O(n), because log M is a constant independent of n. But if we allow this, then there is a much simpler solution: for each integer in the range (all 4294967296 of them), go through the array to see if it occurs more than once. This is also, in a sense, O(n), because 4294967296 is also a constant independent of n.

I don't think my simple solution would count as an answer. But if not, then we shouldn't allow the radix sort, either.