# 99 questions/Solutions/55

### From HaskellWiki

(**) 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.

A slightly more efficient version of this solution, which never creates the same tree twice:

cbalTree 0 = [Empty] cbalTree 1 = [leaf 'x'] cbalTree n = if n `mod` 2 == 1 then [ Branch 'x' l r | l <- cbalTree ((n - 1) `div` 2), r <- cbalTree ((n - 1) `div` 2) ] else concat [ [Branch 'x' l r, Branch 'x' r l] | l <- cbalTree ((n - 1) `div` 2), r <- cbalTree (n `div` 2) ]

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.