4 files changed, 25 insertions, 0 deletions
diff --git a/H55.hs b/H55.hs
index a6e6a95..67e2bbb 100644
--- a/H55.hs
+++ b/H55.hs
@@ -1,3 +1,4 @@
+module H55 where
 import Tree
 
 cbalTree :: Int -> [Tree Char]
diff --git a/H56.hs b/H56.hs
index 3cf7127..c7960f4 100644
--- a/H56.hs
+++ b/H56.hs
@@ -1,3 +1,4 @@
+module H56 where
 import Tree
 
 mirror :: Tree a -> Tree a -> Bool
diff --git a/H57.hs b/H57.hs
new file mode 100644
index 0000000..0af879f
--- /dev/null
+++ b/H57.hs
@@ -0,0 +1,11 @@
+import Tree
+
+construct :: Ord a => [a] -> Tree a
+construct = foldl add Empty
+
+add :: Ord a => Tree a -> a -> Tree a
+add Empty x = Branch x Empty Empty
+add t@(Branch a t1 t2) x = case compare x a of 
+    LT -> Branch a (add t1 x) t2
+    GT -> Branch a t1 (add t2 x)
+    EQ -> t
diff --git a/H58.hs b/H58.hs
new file mode 100644
index 0000000..2176997
--- /dev/null
+++ b/H58.hs
@@ -0,0 +1,12 @@
+import Tree
+import H55
+import H56
+
+symCbalTrees n
+    | n `mod` 2 == 0 = []
+    | otherwise = [Branch 'x' x (reverseTree x) | x <- cbalTree (n `div` 2)]
+
+reverseTree Empty = Empty
+reverseTree (Branch x l r) = Branch x (reverseTree r) (reverseTree l)
+
+symCbalTrees' = filter symmetric . cbalTree