import Data.List

bit i j = big_oh_1_of_accessing_A

get _ [] = []
get k xs
    | length zero < length one = 0 : get (k+1) zero
    | otherwise = 1 : get (k+1) one
        where (zero, one) = partition (\x -> bit x k == 0) xs

missing n = foldl1 (\a b -> a * 2 + b) $ get 0 [0..n-1]