Q:

find kth non-repeating element among characters and integers (large size)

This is a question i found over internet , asked in an interview . 

Given a very large array of character and integers we had to find the K th non repeating element.
  For eg: if the array is  { 1,2,a,s,2,s,b,v }
  and if K=3
  then the answer should be 'b' .

required constraints : O(N) time and O(1) space .
What have i done ? 
The best solution i can come up with is make array of size 256 and hash the elements as they come .
I think this to be a valid solution because size of array 256 is independent of input array size .
I also tried to use Kth-order statistics approach , but could not form an algorithm , because many numbers can be repeated before and after the kth required element , so it's not possible to determine it's location .
What do i  require ? 

Please share your views and how would you approach it.
  I don't want any code . 

A:

Your approach will work only if you consider the alphabet as a constant size, and will fail miserably for unicode32, for example.
Note however that since this is a variation of the element distinctness problem - I believe this problem cannot be solved otherwise within the time and space constraints.
Proof:
Assume you had such an algorithm that uses "real" O(1) space, and O(n) time. Let this algorithm be A.
Given an instance of element distinctness problem, arr - use A(arr,n). The algorithm should return arr[n] (1 based), if and only if every element in the array is unique. Since A is O(n) time and O(1) space - we solved element distinctness problem in O(n) time and O(1) space.

