preorder, postorder, foldTree, mapTree for binary trees

src/BinaryTree/BinarySearchTree.hs

@@ -10,9 +10,17 @@ nodeKey (Node x _ _) = Just x
 
 -- Perform inorder walk of the binary search tree.
 -- Cormen, Thomas H., et al. Introduction to algorithms.  pg. 288, MIT press, 2009.
-inorderWalk :: (Eq a, Ord a) => BTree a -> [a]
-inorderWalk Empty = []
-inorderWalk (Node x l r) = (inorderWalk l) ++ [x] ++ (inorderWalk r)
+inorder :: (Eq a, Ord a) => BTree a -> [a]
+inorder Empty = []
+inorder (Node x l r) = inorder l ++ [x] ++ inorder r
+
+preorder :: BTree a -> [a]
+preorder Empty = []
+preorder (Node a left right) = a : preorder left ++ preorder right
+
+postorder :: BTree a -> [a]
+postorder Empty = []
+postorder (Node a left right) = postorder left ++ postorder right ++ [a]
 
 -- Function to insert a value into the tree. Returns the new tree.
 -- Cormen, Thomas H., et al. Introduction to algorithms.  pg. 294, MIT press, 2009.

src/BinaryTree/BinaryTree.hs

@@ -68,4 +68,17 @@ numNodes t = length $ bfsList t
 -- Pretty Print a Binary Tree
 simplePrint :: (Show a) => BTree a -> String
 simplePrint Empty = ""
-simplePrint t = (nodeShow t) ++ " " ++ (simplePrint $ getLeftTree t) ++ (simplePrint $ getRightTree t)
+simplePrint t = (nodeShow t) ++ " " ++ (simplePrint $ getLeftTree t) ++ (simplePrint $ getRightTree t)
+
+foldTree :: (a -> b -> b) -> b -> BTree a -> b
+foldTree f acc Empty = acc
+foldTree f acc (Node a left right) = foldTree f foldedRightSubtree left where
+    result = f a acc
+    foldedRightSubtree = foldTree f result right
+
+mapTree :: (a -> b) -> BTree a -> BTree b
+mapTree _ Empty = Empty
+mapTree f (Node a left right) = Node (f a) left' right' where
+  left'  = mapTree f left
+  right' = mapTree f right
+