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Construct a complete binary tree

A complete binary tree with height H is defined as follows:

  • The levels 1,2,3,...,H-1 contain the maximum number of nodes (i.e 2**(i-1) at the level i)
  • In level H, which may contain less than the maximum possible number of nodes, all the nodes are "left-adjusted". This means that in a levelorder tree traversal all internal nodes come first, the leaves come second, and empty successors (the nil's which are not really nodes!) come last.

Particularly, complete binary trees are used as data structures (or addressing schemes) for heaps.

We can assign an address number to each node in a complete binary tree by enumerating the nodes in level-order, starting at the root with number 1. For every node X with address A the following property holds: The address of X's left and right successors are 2*A and 2*A+1, respectively, if they exist. This fact can be used to elegantly construct a complete binary tree structure.

Write a predicate complete_binary_tree/2.

import Data.List                                                                                                                                               
 
data Tree a = Empty | Branch a (Tree a) (Tree a)                                                                                                               
        deriving (Show, Eq)                                                                                                                                    
 
filled :: Tree a -> [[Bool]]                                                                                                                                   
filled Empty = repeat [False]                                                                                                                                  
filled (Branch _ l r) = [True] : zipWith (++) (filled l) (filled r)                                                                                            
 
complete_binary_tree :: Int -> Tree Char                                                                                                                       
complete_binary_tree n = generate_tree 1                                                                                                                       
  where generate_tree x                                                                                                                                        
          | x > n     = Empty                                                                                                                                  
          | otherwise = Branch 'x' (generate_tree (2*x)  )                                                                                                     
                                   (generate_tree (2*x+1))                                                                                                     
 
is_complete_binary_tree :: Tree a -> Bool                                                                                                                      
is_complete_binary_tree Empty = True                                                                                                                           
is_complete_binary_tree t = and $ last_proper : zipWith (==) lengths powers                                                                                    
  where levels      = takeWhile or $ filled t                                                                                                                  
        -- The upper levels of the tree should be filled.                                                                                                      
        -- Every level has twice the number of nodes as the one above it,                                                                                      
        -- so [1,2,4,8,16,...]                                                                                                                                 
        lengths     = map (length . filter id) $ init levels                                                                                                   
        powers      = iterate (2*) 1                                                                                                                           
        -- The last level should contain a number of filled spots,                                                                                             
        -- and (maybe) some empty spots, but no filled spots after that!                                                                                       
        last_filled = map head $ group $ last levels                                                                                                           
        last_proper = head last_filled && (length last_filled) < 3

Alternative solution which constructs complete binary trees from a given list using local recursion (also includes a lookup function as per the Prolog solution):

completeBinaryTree :: Int -> a -> Tree a
completeBinaryTree n = cbtFromList . replicate n
 
cbtFromList :: [a] -> Tree a
cbtFromList xs = let (t, xss) = cbt (xs:xss) in t
  where cbt ((x:xs):xss) =
                let (l, xss') = cbt xss
                    (r, xss'') = cbt xss'
                in  (Branch x l r, xs:xss'')
        cbt _ = (Empty, [])
 
lookupIndex :: Tree a -> Integer -> a
lookupIndex t = lookup t . path
    where lookup Empty _ = error "index to large"
          lookup (Branch x _ _) [] = x
          lookup (Branch x l r) (p:ps) = lookup (if even p then l else r) ps
 
          path = reverse . takeWhile (>1) . iterate (`div` 2) . (1+)