Remove from the array elements that are repeated

Question

What is the best way to remove from the array elements that are repeated. For example, from the array

a = [4, 3, 3, 1, 6, 6]

need to get

Answer 1

a = [4, 3, 3, 1, 6, 6]
a.select{|b| a.count(b) == 1}
#=> [4, 1]

More complicated but faster solution (O(n) I believe :))

a = [4, 3, 3, 1, 6, 6]
ar = []
add = proc{|to, form| to << from[1] if form.uniq.size == from.size }
a.sort!.each_cons(3){|b| add.call(ar, b)}
ar << a[0] if a[0] != a[1]; ar << a[-1] if a[-1] != a[-2]