September 2017 Update

Task/Tree-traversal/Haskell/tree-traversal.hs

@@ -1,45 +1,55 @@
-data Tree a = Empty
-            | Node { value :: a,
-                     left  :: Tree a,
-                     right :: Tree a }
+data Tree a
+  = Empty
+  | Node { value :: a
+         , left :: Tree a
+         , right :: Tree a}
 
 preorder, inorder, postorder, levelorder :: Tree a -> [a]
+preorder Empty = []
+preorder (Node v l r) = v : preorder l ++ preorder r
 
-preorder Empty        = []
-preorder (Node v l r) =    [v]
-                        ++ preorder l
-                        ++ preorder r
+inorder Empty = []
+inorder (Node v l r) = inorder l ++ (v : inorder r)
 
-inorder Empty        = []
-inorder (Node v l r) =    inorder l
-                       ++ [v]
-                       ++ inorder r
-
-postorder Empty        = []
-postorder (Node v l r) =    postorder l
-                         ++ postorder r
-                         ++ [v]
+postorder Empty = []
+postorder (Node v l r) = postorder l ++ postorder r ++ [v]
 
 levelorder x = loop [x]
-    where loop []                = []
-          loop (Empty      : xs) = loop xs
-          loop (Node v l r : xs) = v : loop (xs ++ [l,r])
+  where
+    loop [] = []
+    loop (Empty:xs) = loop xs
+    loop (Node v l r:xs) = v : loop (xs ++ [l, r])
 
+-- TEST --------------------------------------------------------------
 tree :: Tree Int
-tree = Node 1
-            (Node 2
-                  (Node 4
-                        (Node 7 Empty Empty)
-                        Empty)
-                  (Node 5 Empty Empty))
-            (Node 3
-                  (Node 6
-                        (Node 8 Empty Empty)
-                        (Node 9 Empty Empty))
-                  Empty)
+tree =
+  Node
+    1
+    (Node 2 (Node 4 (Node 7 Empty Empty) Empty) (Node 5 Empty Empty))
+    (Node 3 (Node 6 (Node 8 Empty Empty) (Node 9 Empty Empty)) Empty)
 
+asciiTree :: String
+asciiTree =
+  unlines
+    [ "         1"
+    , "        / \\"
+    , "       /   \\"
+    , "      /     \\"
+    , "     2       3"
+    , "    / \\     /"
+    , "   4   5   6"
+    , "  /       / \\"
+    , " 7       8   9"
+    ]
+
+-- OUTPUT --------------------------------------------------------------
 main :: IO ()
-main = do print $ preorder tree
-          print $ inorder tree
-          print $ postorder tree
-          print $ levelorder tree
+main = do
+  putStrLn asciiTree
+  mapM_ putStrLn $
+    zipWith
+      (\s xs -> justifyLeft 14 ' ' (s ++ ":") ++ unwords (show <$> xs))
+      ["preorder", "inorder", "postorder", "level-order"]
+      ([preorder, inorder, postorder, levelorder] <*> [tree])
+  where
+    justifyLeft n c s = take n (s ++ replicate n c)