commit 739ea0d1add7ffb25b3ce8a71cc9e3c8f506feca
Author: Danila Fedorin <danila.fedorin@gmail.com>
Date:   Fri Jan 11 21:17:12 2019 -0800

    Add solutions to homework 1.

diff --git a/HW1.fedorind.hs b/HW1.fedorind.hs
new file mode 100644
index 0000000..4e52e8c
--- /dev/null
+++ b/HW1.fedorind.hs
@@ -0,0 +1,217 @@
+-- Author: Danila Fedorin, Thursday, January 10
+module HW1 where
+
+
+-- | Integer-labeled binary trees.
+data Tree
+   = Node Int Tree Tree   -- ^ Internal nodes
+   | Leaf Int             -- ^ Leaf nodes
+  deriving (Eq,Show)
+
+-- | An example binary tree, which will be used in tests.
+t1 :: Tree
+t1 = Node 1 (Node 2 (Node 3 (Leaf 4) (Leaf 5))
+                    (Leaf 6))
+            (Node 7 (Leaf 8) (Leaf 9))
+
+-- | Another example binary tree, used in tests.
+t2 :: Tree
+t2 = Node 6 (Node 2 (Leaf 1) (Node 4 (Leaf 3) (Leaf 5)))
+            (Node 8 (Leaf 7) (Leaf 9))
+
+-- | Right associative fold.
+-- The best way here would be to add a foldable instance for Tree.
+-- however, since Tree doesn't take a type parameter, this is not possible.
+-- We settle for these functions.
+treeFoldr :: (Int -> a -> a) -> a -> Tree -> a
+treeFoldr f a (Leaf i) = f i a
+treeFoldr f a (Node i l r) = treeFoldr f (f i (treeFoldr f a r)) l
+
+-- | foldr for non-empty lists.
+treeFoldr1 :: (Int -> Int -> Int) -> Tree -> Int
+treeFoldr1 f (Leaf i) = i
+treeFoldr1 f (Node i l r) = treeFoldr f (f i (treeFoldr1 f r)) l
+
+-- | Left associative fold.
+treeFoldl :: (Int -> a -> a) -> a -> Tree -> a
+treeFoldl f a (Leaf i) = f i a
+treeFoldl f a (Node i l r) = treeFoldl f (f i (treeFoldl f a l)) r
+
+-- | foldl for non-empty lists.
+treeFoldl1 :: (Int -> Int -> Int) -> Tree -> Int
+treeFoldl1 f (Leaf i) = i
+treeFoldl1 f (Node i l r) = treeFoldl f (f i (treeFoldl1 f l)) r
+
+-- | In-order traversal fold.
+treeFold :: (Int -> a -> a) -> a -> Tree -> a
+treeFold f a (Leaf i) = f i a
+treeFold f a (Node i l r) = f i $ treeFold f (treeFold f a r) l
+
+-- | The integer at the left-most node of a binary tree.
+--
+--   >>> leftmost (Leaf 3)
+--   3
+--
+--   >>> leftmost (Node 5 (Leaf 6) (Leaf 7))
+--   6
+--   
+--   >>> leftmost t1
+--   4
+--
+--   >>> leftmost t2
+--   1
+--
+leftmost :: Tree -> Int
+leftmost = treeFoldr1 (\i _ -> i)
+
+-- | The integer at the right-most node of a binary tree.
+--
+--   >>> rightmost (Leaf 3)
+--   3
+--
+--   >>> rightmost (Node 5 (Leaf 6) (Leaf 7))
+--   7
+--   
+--   >>> rightmost t1
+--   9
+--
+--   >>> rightmost t2
+--   9
+--
+rightmost :: Tree -> Int
+rightmost = treeFoldl1 (\i _ -> i)
+
+-- | Get the maximum integer from a binary tree.
+--
+--   >>> maxInt (Leaf 3)
+--   3
+--
+--   >>> maxInt (Node 5 (Leaf 4) (Leaf 2))
+--   5
+--
+--   >>> maxInt (Node 5 (Leaf 7) (Leaf 2))
+--   7
+--
+--   >>> maxInt t1
+--   9
+--
+--   >>> maxInt t2
+--   9
+--
+maxInt :: Tree -> Int
+maxInt = treeFoldr1 max
+
+
+-- | Get the minimum integer from a binary tree.
+--
+--   >>> minInt (Leaf 3)
+--   3
+--
+--   >>> minInt (Node 2 (Leaf 5) (Leaf 4))
+--   2
+--
+--   >>> minInt (Node 5 (Leaf 4) (Leaf 7))
+--   4
+--
+--   >>> minInt t1
+--   1
+--
+--   >>> minInt t2
+--   1
+--
+minInt :: Tree -> Int
+minInt = treeFoldr1 min
+
+
+-- | Get the sum of the integers in a binary tree.
+--
+--   >>> sumInts (Leaf 3)
+--   3
+--
+--   >>> sumInts (Node 2 (Leaf 5) (Leaf 4))
+--   11
+--
+--   >>> sumInts t1
+--   45
+--
+--   >>> sumInts (Node 10 t1 t2)
+--   100
+--
+sumInts :: Tree -> Int
+sumInts = treeFoldr1 (+)
+
+
+-- | The list of integers encountered by a pre-order traversal of the tree.
+--
+--   >>> preorder (Leaf 3)
+--   [3]
+--
+--   >>> preorder (Node 5 (Leaf 6) (Leaf 7))
+--   [5,6,7]
+--
+--   >>> preorder t1
+--   [1,2,3,4,5,6,7,8,9]
+--
+--   >>> preorder t2
+--   [6,2,1,4,3,5,8,7,9]
+--   
+preorder :: Tree -> [ Int ]
+preorder = treeFold (:) []
+
+-- | The list of integers encountered by an in-order traversal of the tree.
+--
+--   >>> inorder (Leaf 3)
+--   [3]
+--
+--   >>> inorder (Node 5 (Leaf 6) (Leaf 7))
+--   [6,5,7]
+--
+--   >>> inorder t1
+--   [4,3,5,2,6,1,8,7,9]
+--
+--   >>> inorder t2
+--   [1,2,3,4,5,6,7,8,9]
+--   
+inorder :: Tree -> [ Int ]
+inorder = treeFoldr (:) []
+
+
+-- | Check whether a binary tree is a binary search tree.
+--
+--   >>> isBST (Leaf 3)
+--   True
+--
+--   >>> isBST (Node 5 (Leaf 6) (Leaf 7))
+--   False
+--   
+--   >>> isBST t1
+--   False
+--
+--   >>> isBST t2
+--   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
+
+
+-- | Check whether a number is contained in a binary search tree.
+--   (You may assume that the given tree is a binary search tree.)
+--
+--   >>> inBST 2 (Node 5 (Leaf 2) (Leaf 7))
+--   True
+--
+--   >>> inBST 3 (Node 5 (Leaf 2) (Leaf 7))
+--   False
+--
+--   >>> inBST 4 t2
+--   True
+--
+--   >>> inBST 10 t2
+--   False
+--   
+inBST :: Int -> Tree -> Bool
+inBST i = treeFoldr (\v acc -> acc || (v == i)) False