# Difference between revisions of "99 questions/Solutions/72"

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## Latest revision as of 03:52, 10 January 2017

(*) Construct the bottom-up order sequence of the tree nodes.

Write a predicate bottom_up(Tree,Seq) which constructs the bottom-up sequence of the nodes of the multiway tree Tree.

```
bottom_up :: Tree a -> [a]
bottom_up (Node x ts) = concatMap bottom_up ts ++ [x]
```

A more efficient version using an accumulator:

```
bottom_up :: Tree a -> [a]
bottom_up t = bottom_up_aux t []
where bottom_up_aux :: Tree a -> [a] -> [a]
bottom_up_aux (Node x ts) xs = foldr bottom_up_aux (x:xs) ts
```