Mind boggling card trick: Difference between revisions

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 import Data.Monoid ((<>))
--- RED vs BLACK ----------------------------------------
-rb :: Int -> Char
-rb n
-  | even n = 'R'
-  | otherwise = 'B'
--- THREE STACKS ----------------------------------------
-threeStacks :: String -> (String, String, String)
-threeStacks cards =
-  let cut = quot (length cards) 2
-      discards = take cut cards
-      pairs = (zip discards $ drop cut cards)
-      (rs, bs) = partition (('R' ==) . fst) pairs
-  in (snd <$> rs, snd <$> bs, discards)
--- TEST ------------------------------------------------
 main :: IO [Int]
 main = do
@@ Line 306: / Line 290: @@
       ]
   return ns
+-- RED vs BLACK ----------------------------------------
+rb :: Int -> Char
+rb n
+  | even n = 'R'
+  | otherwise = 'B'
+-- THREE STACKS ----------------------------------------
+threeStacks :: String -> (String, String, String)
+threeStacks cards = go ([],[],[]) cards
+  where
+    go tpl [] = tpl
+    go (rs, bs, ds) (x:[]) = (rs, bs, x:ds)
+    go (rs, bs, ds) (x: y: xs)
+      | 'R' == x = go (y:rs, bs, x:ds) xs
+      | otherwise = go (rs, y:bs, x:ds) xs
 -- SHUFFLE -----------------------------------------------