From baa7ab145907c7959561bafdd667e1bac1722cef Mon Sep 17 00:00:00 2001
From: Danila Fedorin <danila.fedorin@gmail.com>
Date: Fri, 18 Jan 2019 11:16:19 -0800
Subject: [PATCH] Add fold-based solution for isBST. Thanks random guy in the
 front for the idea.

---
 HW1.fedorind.hs | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/HW1.fedorind.hs b/HW1.fedorind.hs
index 6456dca..0cc2240 100644
--- a/HW1.fedorind.hs
+++ b/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.)