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

The approach that would come closest to O(N) in time is probably a conventional hash table, where the hash entries are simply the numbers, used as keys. You'd walk through the list, inserting each entry in the hash table, after first checking whether it was already in the table.

Not strictly O(N), however, since hash search/insertion gets slower as the table fills up. And in terms of storage it would be expensive for large lists -- at least 3x and possibly 10-20x the size of the array of numbers.