Two sum: Difference between revisions

Or, using concatMap and nubBy:

<lang haskell>import Data.List (elem, elemIndices, nubBy)

import Control.Monad (join)

summingPairIndices :: Int -> [Int] -> [[Int]]

summingPairIndices n xs =

(join . (flip elemIndices xs <$>)) <$>

nubBy

(\[x, y] zs -> and (flip elem zs <$> [x, y]))

(concatMap

(\[x, y] ->

if x == y

then []

else [ [x, y]

| x + y == n ]) $

sequence [xs, xs])

main :: IO ()

main = print (summingPairIndices 21 [0, 2, 11, 19, 90])</lang>

{{Out}}

<pre>[[1,3]]</pre>