Solution for [Filter Elements] - https://www.hackerrank.com/challenges/filter-elements

a_at_least_k.hs

module hackerrank where

import Data.List

orderedElem :: [Int] -> [Int]
orderedElem = foldl (\r x -> if x `elem` r then r else r++[x]) []

unorderedGrouping :: [Int] -> [[Int]]
unorderedGrouping = group . sort

orderedGrouping :: [Int] -> [[Int]]
orderedGrouping = (\(es, gs) -> (es >>= (\e -> filter (\(x:_) -> x == e) gs))) . apply2 
    where apply2 = (,) <$> orderedElem <*> unorderedGrouping

result :: [Int] -> Int -> [Int]
result a k = check $ filter ((>= k) . length) $ orderedGrouping a
    where check n   | n == [] = [-1]
                    | otherwise = map head n