Odd Occurrences In Array

Haskell Odd Occurrences In Array

import Data.Bits (xor)

oddOccurrencesInArray :: [Int] -> Maybe Int
oddOccurrencesInArray []      = Nothing
oddOccurrencesInArray a       = Just (foldl1 xor a)

This uses XOR to cancel out pairs, leaving only the value that appears an odd number of times.