Two sum: Difference between revisions
→{{header|Haskell}}: Alternative approach, using concatMap and nubBy
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[1,3]
</pre>
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>
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