add 2.1 remove dup in the linked list

Author: cd155

src/LinkedList.hs

@@ -6,6 +6,47 @@ import Basics (Nat)
 -- A sequence of nodes, simple very of graph
 data LinkedList a = Null'| Node' a (LinkedList a) deriving Show
 
+instance Eq a => Eq (LinkedList a) where
+  Null' == Null' = True
+  Null' == (Node' _ _) = False
+  (Node' _ _) == Null' = False
+  (Node' x1 x2) == (Node' y1 y2) = x1 == y1 && x2 == y2
+
+testLink = Node' 1 $ Node' 2 $ Node' 1 $ Node' 5 $ Node' 2 $ Node' 3 Null'
+
+{-
+    2.1
+    Write code to remove duplicates from an unsorted linked list.
+
+    Test Case:
+        removeDup testLink
+-}
+removeDup :: Eq a => LinkedList a -> LinkedList a
+removeDup xs = removeDupHelper xs []
+
+removeDupHelper :: Eq a => LinkedList a -> [LinkedList a] -> LinkedList a
+removeDupHelper Null' _ = Null'
+removeDupHelper (Node' x y) dict
+    | Node' x Null' `elem` dict = removeDupHelper y dict
+    | otherwise = Node' x (removeDupHelper y (Node' x Null':dict))
+
+-- (no dict)loop into the original list, and refine the result list
+removeDupBF :: Eq a => LinkedList a -> LinkedList a
+removeDupBF xs = removeDupBFHelper xs xs
+
+removeDupBFHelper :: Eq a => LinkedList a -> LinkedList a -> LinkedList a
+removeDupBFHelper Null' refined = refined
+removeDupBFHelper _ Null' = Null'
+removeDupBFHelper (Node' x l1) (Node' y l2) = 
+    removeDupBFHelper l1 (refineLink (Node' x Null') (Node' y l2) True)
+
+refineLink :: Eq a => LinkedList a -> LinkedList a -> Bool -> LinkedList a
+refineLink _ Null' _ = Null'
+refineLink x (Node' y l) isFirst
+    | (x == Node' y Null') && not isFirst = refineLink x l isFirst
+    | (x == Node' y Null') && isFirst = Node' y (refineLink x l False)
+    | otherwise = Node' y (refineLink x l isFirst)
+
 {-
     4.3 Given a binary tree, design an algorithm which creates 
     a linked list of all the nodes at each depth.
@@ -35,6 +76,6 @@ divideTreeHelper xs track (Just y:ys) =
 -- Check whether it is belong to the set of power of two
 isPowerOfTwo :: Nat -> Bool
 isPowerOfTwo 1 = True
-isPowerOfTwo x 
+isPowerOfTwo x
     | x `mod` 2 == 1 = False
     | otherwise = isPowerOfTwo (x `div` 2)