# 99 questions/Solutions/60

### From HaskellWiki

(**) Construct height-balanced binary trees with a given number of nodes

Consider a height-balanced binary tree of height H. What is the maximum number of nodes it can contain?

Clearly, MaxN = 2**H - 1. However, what is the minimum number MinN? This question is more difficult. Try to find a recursive statement and turn it into a functionminNodes

maxHeight

Now, we can attack the main problem: construct all the height-balanced binary trees with a given nuber of nodes. Find out how many height-balanced trees exist for N = 15.

hbalTreeNodes _ 0 = [Empty] hbalTreeNodes x n = concatMap toFilteredTrees [minHeight .. maxHeight] where toFilteredTrees = filter ((n ==) . countNodes) . hbalTree x -- Similar to the Fibonacci sequence but adds 1 in each step. minNodesSeq = 0:1:zipWith ((+).(1+)) minNodesSeq (tail minNodesSeq) minNodes = (minNodesSeq !!) minHeight = ceiling $ logBase 2 $ fromIntegral (n+1) maxHeight = (fromJust $ findIndex (>n) minNodesSeq) - 1 countNodes Empty = 0 countNodes (Branch _ l r) = countNodes l + countNodes r + 1

Another solution generates only the trees we want:

-- maximum number of nodes in a weight-balanced tree of height h maxNodes :: Int -> Int maxNodes h = 2^h - 1 -- minimum height of a weight-balanced tree of n nodes minHeight :: Int -> Int minHeight n = ceiling $ logBase 2 $ fromIntegral (n+1) -- minimum number of nodes in a weight-balanced tree of height h minNodes :: Int -> Int minNodes h = fibs !! (h+2) - 1 -- maximum height of a weight-balanced tree of n nodes maxHeight :: Int -> Int maxHeight n = length (takeWhile (<= n+1) fibs) - 3 -- Fibonacci numbers fibs :: [Int] fibs = 0 : 1 : zipWith (+) fibs (tail fibs) hbalTreeNodes :: a -> Int -> [Tree a] hbalTreeNodes x n = [t | h <- [minHeight n .. maxHeight n], t <- baltree h n] where -- baltree h n = weight-balanced trees of height h with n nodes -- assuming minNodes h <= n <= maxNodes h baltree 0 n = [Empty] baltree 1 n = [Branch x Empty Empty] baltree h n = [Branch x l r | (hl,hr) <- [(h-2,h-1), (h-1,h-1), (h-1,h-2)], let min_nl = max (minNodes hl) (n - 1 - maxNodes hr), let max_nl = min (maxNodes hl) (n - 1 - minNodes hr), nl <- [min_nl .. max_nl], let nr = n - 1 - nl, l <- baltree hl nl, r <- baltree hr nr]