Add basic (fast) solutions to HW2
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HW2.fedorind.hs
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103
HW2.fedorind.hs
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module HW2 where
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-- | Binary trees with nodes labeled by values of an arbitrary type.
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data Tree a
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= Node a (Tree a) (Tree a)
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| End
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deriving (Eq,Show)
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-- | One step in a path, indicating whether to follow the left subtree (L)
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-- or the right subtree (R).
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data Step = L | R
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deriving (Eq,Show)
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-- | A path is a sequence of steps. Each node in a binary tree can be
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-- identified by a path, indicating how to move down the tree starting
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-- from the root.
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type Path = [Step]
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-- | Create a leaf node.
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leaf :: a -> Tree a
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leaf x = Node x End End
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-- | An example tree.
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ex :: Tree Int
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ex = Node 4 (Node 3 (leaf 2) End)
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(Node 7 (Node 5 End (leaf 6))
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(leaf 8))
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instance Functor Tree where
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fmap f End = End
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fmap f (Node v l r) = Node (f v) (fmap f l) (fmap f r)
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-- | Map a function over a tree. Applies the given function to every label
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-- in the tree, preserving the tree's structure.
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--
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-- >>> mapTree odd End
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-- End
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--
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-- >>> mapTree even (Node 5 (leaf 2) End)
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-- Node False (Node True End End) End
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--
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-- >>> (mapTree not . mapTree even) (Node 5 End (leaf 2))
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-- Node True End (Node False End End)
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--
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-- >>> mapTree (+10) ex
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-- Node 14 (Node 13 (Node 12 End End) End) (Node 17 (Node 15 End (Node 16 End End)) (Node 18 End End))
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--
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-- >>> ex == (mapTree (subtract 27) . mapTree (+27)) ex
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-- True
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--
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mapTree :: (a -> b) -> Tree a -> Tree b
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mapTree = fmap
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-- | Get the value at the node specified by a path. Returns 'Nothing' if
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-- the given path is invalid.
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--
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-- >>> valueAt [] ex
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-- Just 4
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--
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-- >>> valueAt [L,L] ex
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-- Just 2
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--
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-- >>> valueAt [L,R] ex
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-- Nothing
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--
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-- >>> valueAt [R,L,R] ex
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-- Just 6
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--
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-- >>> valueAt [L,L,L] ex
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-- Nothing
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--
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valueAt :: Path -> Tree a -> Maybe a
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valueAt _ End = Nothing
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valueAt [] (Node a _ _) = Just a
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valueAt (x:xs) (Node a l r) = valueAt xs $ if x == L then l else r
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-- | Find a path to a node that contains the given value.
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--
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-- >>> pathTo 3 (leaf 5)
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-- Nothing
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--
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-- >>> pathTo 5 ex
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-- Just [R,L]
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--
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-- >>> pathTo 6 ex
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-- Just [R,L,R]
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--
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-- >>> pathTo 4 ex
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-- Just []
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--
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-- >>> pathTo 10 ex
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-- Nothing
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--
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pathTo :: Eq a => a -> Tree a -> Maybe Path
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pathTo v End = Nothing
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pathTo v (Node a l r) = orElse currentNode $ orElse (pathHelper v l L) $ pathHelper v r R
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where
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currentNode = if a == v then Just [] else Nothing
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pathHelper v tree dir = (pathTo v tree) >>= (Just . (dir:))
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orElse m1 m2 = case m1 of
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Just _ -> m1
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Nothing -> m2
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