Find three numbers appeared only once

Question

In a sequence of length n, where n=2k+3, that is there are k unique numbers appeared twice and three numbers appeared only once.

The question is: how to fin

Answer 1

If a probabilistic solution will suffice then you could use a Bloom Filter.

Create two Bloom filters. The first (A) contains numbers that have been found at least one, and the second (B) contains numbers that have been found twice.

Pseudocode:

A = empty
B = empty

foreach x in the list
  if x in A
    add x to B
  else
    add x to A

foreach x in the list
  if x in A
    if !(x in B)
      print x

If you use the full 1000KB then the probability of error would be ridiculously low.