Tree.hs

import Data.List (elemIndex)
import Data.Maybe (fromJust)

--similar to /u/Ashandalar's definition, but more general just for kicks 
-----
data Node a = Node a [Node a] deriving (Show)

-- Wrapper around makeNode. Converts the list to index/value pairs
-- so it's easier to work with. So, [4,0,0,-1,3] becomes .
-- Given [4,0,0,-1,3], returns (makeNode [(0,4),(1,0),(2,0),(3,-1),(4,3)] (3,-1))
-----
fromParentIxs :: [Int] -> Node Int 
fromParentIxs ls = makeNode zippedLs (fromJust $ elemIndex (-1) ls, -1)  
    where zippedLs = zip [0..] ls

-- Simply create a node given a tuple. To create the child list, we just map this 
-- function over the subset of tuples for which the "parent pointer" (a.k.a. the 
-- second item in the tuple) is the index of this node. Notice we don't even need
-- the "parent pointer" of the node we're currently working on.
-----
makeNode :: Eq a => [(a,a)] -> (a,a) -> Node a 
makeNode [] (i, _) = Node i []
makeNode ls (i, _) = Node i children
    where children    = map (makeNode ls) childTuples
          childTuples = filter (\(_,parent) -> parent == i) ls