September 2017 Update

Task/Equilibrium-index/Haskell/equilibrium-index-4.hs

@@ -0,0 +1,27 @@
+equilibriumIndices :: [Int] -> [Int]
+equilibriumIndices xs =
+  foldr
+    (\(x, y, i) a ->
+        (if x == y
+           then i : a
+           else a))
+    []
+    (zip3
+       (scanl1 (+) xs) -- Sums from the left
+       (scanr1 (+) xs) -- Sums from the right
+       [0 ..] -- Indices
+     )
+
+-- TEST -----------------------------------------------------------------------
+main :: IO ()
+main =
+  mapM_
+    print
+    (equilibriumIndices <$>
+     [ [-7, 1, 5, 2, -4, 3, 0]
+     , [2, 4, 6]
+     , [2, 9, 2]
+     , [1, -1, 1, -1, 1, -1, 1]
+     , [1]
+     , []
+     ])