Add fold-based solution for isBST.

Thanks random guy in the front for the idea.

HW1.fedorind.hs

@@ -191,12 +191,7 @@ inorder = treeFoldr (:) []
 --   True
 --
 isBST :: Tree -> Bool
-isBST (Leaf _) = True
-isBST (Node i l r) = i >= value l && i <= value r && isBST l && isBST r
-    where
-        value (Leaf i) = i
-        value (Node i _ _) = i
-
+isBST tree = snd $ treeFoldl (\v (p, b) -> (v, b && v >= p)) (minInt tree, True) tree
 
 -- | Check whether a number is contained in a binary search tree.
 --   (You may assume that the given tree is a binary search tree.)