λ> pickMajorityUnsafe [3,1,2,2,1,3]
3
λ> pickMajorityUnsafe [3,1,2,2,1]
1
λ> pickMajorityUnsafe [3,2,2,1]
2

import Data.List
import Data.Monoid
import Data.Ord

pickMajorityUnsafe :: (Ord a) => [a] -> a
pickMajorityUnsafe xs = head
                      . head
                      . sortBy (flip (comparing length) <> comparing originalIndex)
                      . group
                      . sort
                      $ xs
  where
    originalIndex gs = head gs `elemIndex` xs