Algorithm to find a duplicate entry in constant space and O(n) time

Question

Given an array of N integer such that only one integer is repeated. Find the repeated integer in O(n) time and constant space. There is no range for the value of integers or

Answer 1

As was already mentioned by others, I don't see any way to do it in O(n).

However, you can try a probabilistic approach by using a Bloom Filter. It will give you O(n) if you are lucky.