(**) Construct completely balanced binary trees
In a completely balanced binary tree, the following property holds for every node: The number of nodes in its left subtree and the number of nodes in its right subtree are almost equal, which means their difference is not greater than one.
Write a function cbal-tree to construct completely balanced binary trees for a given number of nodes. The predicate should generate all solutions via backtracking. Put the letter 'x' as information into all nodes of the tree.
cbalTree :: Int -> [Tree Char] cbalTree 0 = [Empty] cbalTree n = let (q, r) = (n - 1) `quotRem` 2 in [Branch 'x' left right | i <- [q .. q + r], left <- cbalTree i, right <- cbalTree (n - i - 1)]
This solution uses a list comprehension to enumerate all the trees, in a style that is more natural than standard backtracking.
The base case is a tree of size 0, for which Empty is the only possibility. Trees of size n == 1 or larger consist of a branch, having left and right subtrees with sizes that sum up to n - 1. This is accomplished by getting the quotient and remainder of (n - 1) divided by two; the remainder will be 0 if n is odd, and 1 if n is even. For n == 4, (q, r) = (1, 1).
Inside the list comprehension, i varies from q to q + r. In our n == 4 example, i will vary from 1 to 2. We recursively get all possible left subtrees of size [1..2], and all right subtrees with the remaining elements.
When we recursively call cbalTree 1, q and r will both be 0, thus i will be 0, and the left subtree will simply be Empty. The same goes for the right subtree, since n - i - 1 is 0. This gives back a branch with no children--a "leaf" node:
> cbalTree 1 [Branch 'x' Empty Empty]
The call to cbalTree 2 sets (q, r) = (0, 1), so we'll get back a list of two possible subtrees. One has an empty left branch, the other an empty right branch:
> cbalTree 2 [ Branch 'x' Empty (Branch 'x' Empty Empty), Branch 'x' (Branch 'x' Empty Empty) Empty ]
In this way, balances trees of any size can be built recursively from smaller trees.
Another approach is to create a list of all possible tree structures with a given number of nodes, and then filter that list on whether or not the tree is balanced.
data Tree a = Empty | Branch a (Tree a) (Tree a) deriving (Show, Eq) leaf x = Branch x Empty Empty main = putStrLn $ concatMap (\t -> show t ++ "\n") balTrees where balTrees = filter isBalancedTree (makeTrees 'x' 4) isBalancedTree :: Tree a -> Bool isBlanacedTree Empty = True isBalancedTree (Branch _ l r) = abs (countBranches l - countBranches r) ≤ 1 && isBalancedTree l && isBalancedTree r isBalancedTree _ = False countBranches :: Tree a -> Int countBranches Empty = 0 countBranches (Branch _ l r) = 1 + countBranches l + countBranches r -- makes all possible trees filled with the given number of nodes -- and fill them with the given value makeTrees :: a -> Int -> [Tree a] makeTrees _ 0 =  makeTrees c 1 = [leaf c] makeTrees c n = lonly ++ ronly ++ landr where lonly = [Branch c t Empty | t <- smallerTree] ronly = [Branch c Empty t | t <- smallerTree] landr = concat [[Branch c l r | l <- fst lrtrees, r <- snd lrtrees] | lrtrees <- treeMinusTwo] smallerTree = makeTrees c (n-1) treeMinusTwo = [(makeTrees c num, makeTrees c (n-1-num)) | num <- [0..n-2]]
While not nearly as neat as the previous solution, this solution uses some generic binary tree methods that could be useful in other contexts.