## Binary Tree Traversal

### October 18, 2013

In a previous exercise, we wrote a small library for maintaining binary search trees, including a function that traversed the tree in order. In today’s exercise we will write functions that traverse a tree in pre-order and post-order.

Given the tree shown at right, a pre-order traversal visits the node in this order: 8 3 1 6 4 7 10 14 13. At each level of the tree, pre-order traversal first handles the current node, then calls itself recursively on the left child, then calls itself recursively on the right child.

A post-order traversal visits the nodes in this order: 1 4 7 6 3 13 14 10 8. At each level of the tree, post-order traversal calls itself recursively on the left child, then calls itself recursively on the right child, then finally handles the current node.

In addition to traversing a tree in pre-order or post-order, you should write functions that reconstruct the original tree given a list of the tree nodes in pre-order or post-order.

Your task is to write the four functions described above. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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### 9 Responses to “Binary Tree Traversal”

1. Maxime said

My solution for the two first functions, in 105 bytes of JavaScript:

// ordering function
// parameters: source tree, destination array, direction (pre:1, post: 0)
o=function(b,c,d){void 0!==b&&(isNaN(b)?(d&&c.push(b[1]),o(b[0],c),o(b[2],c),d||c.push(b[1])):c.push(b))}

// tests
tree = [[1,3,[4,6,7]],8,[,10,[13,14,]]];
pre_order_array = [];
post_order_array = [];

o(tree, pre_order_array, 1);
console.log(pre_order_array); // -> [8, 1, 4, 7, 6, 3, 13, 14, 10]

o(tree, post_order_array, 0);
console.log(post_order_array); // -> [1, 4, 7, 6, 3, 13, 14, 10, 8]

2. Maxime said

Sorry, I meant:

o=function(b,c,d){void 0!==b&&(isNaN(b)?(d&&c.push(b[1]),o(b[0],c,d),o(b[2],c,d),d||c.push(b[1])):c.push(b))}

This function returns a correct array for pre-order.

3. Peter Salvi said

Common Lisp solution:

```;;; Node; (VALUE LEFT RIGHT)

(defun preorder (tree)
(when tree
(cons (first tree) (append (preorder (second tree)) (preorder (third tree))))))

(defun postorder (tree)
(when tree
(append (postorder (second tree)) (postorder (third tree)) (list (first tree)))))

(defun tree-insert (tree a)
(if (null tree)
(list a nil nil)
(cond ((= a (first tree)) tree)
((< a (first tree))
(list (first tree) (tree-insert (second tree) a) (third tree)))
(t (list (first tree) (second tree) (tree-insert (third tree) a))))))

(defun tree-from-preorder (lst)
(reduce #'tree-insert lst :initial-value nil))

(defun tree-from-postorder (lst)
(reduce #'tree-insert (reverse lst) :initial-value nil))
```
4. ```structure ListX = struct
fun span f []      = ([], [])
| span f (x::xs) =
if f x
then
let
val (ys, zs) = span f xs
in
(x::ys, zs)
end
else
([], x::xs)

fun splitAtLast []      = raise List.Empty
| splitAtLast [x]     = ([], x)
| splitAtLast (x::xs) =
let
val (ys, y) = splitAtLast xs
in
(x::ys, y)
end
end

datatype 'a tree =
Tree of 'a * 'a tree * 'a tree
| MTTree

fun traverse f g e MTTree           = e
| traverse f g e (Tree (a, l, r)) =
g (f a) (traverse f g e l) (traverse f g e r)

fun pre t =
let
fun g e ls rs = e::(ls@rs)
in
traverse (fn x => x) g [] t
end

fun post t =
let
fun g e ls rs = (ls@rs)@[e]
in
traverse (fn x => x) g [] t
end

datatype traversal = Pre | Post

local
fun root Pre  (x::xs) = (xs, x)
| root Post xs      = ListX.splitAtLast xs
in
fun recons _ [] = MTTree
| recons t xs =
let
val (ys, y) = root t xs
val (l, r)  = ListX.span (fn a => a < y) ys
in
Tree (y, recons t l, recons t r)
end
end

val a = pre (recons Pre [8, 3, 1, 6, 4, 7, 10, 14, 13])
= [8, 3, 1, 6, 4, 7, 10, 14, 13]
val b = post (recons Post [1, 4, 7, 6, 3, 13, 14, 10, 8])
= [1, 4, 7, 6, 3, 13, 14, 10, 8]
```
5. Josef Svenningsson said

For the task of recreating the tree from a preorder traversal I managed to come up with a function which traverses the list once and doesn’t allocate anything extra except the tree (well, it does allocate tuples, but they can be removed). So that’s quite nice, but I failed to do the same thing with the postorder traversal.

``````fromPreOrder :: Ord a => [a] -> Tree a
fromPreOrder [] = Leaf
fromPreOrder (a:as) = Branch a l (fromPreOrder bs)
where
(l,bs) = lessThan a as

lessThan n [] = (Leaf,[])
lessThan n all@(a:as)
| a >= n    = (Leaf,all)
| otherwise = (Branch a l r,cs)
where (l,bs) = lessThan a as
(r,cs) = lessThan n bs

``````
6. Marc Young said

In xquery:

declare function local:post-order(
\$tree as node()
) {
let \$seq :=
if (local:has-children(\$tree)) then (
let \$left := \$tree/node()[1]
let \$right := \$tree/node()[2]
return
(
local:post-order(\$left),
local:post-order(\$right),
\$tree/@val/fn:string()
)
) else (
\$tree/@val/fn:string()
)
return \$seq
};

declare function local:pre-order(
\$tree as node()
) {
let \$seq :=
if (local:has-children(\$tree)) then (
let \$left := \$tree/node()[1]
let \$right := \$tree/node()[2]
return
(
\$tree/@val/fn:string(),
local:pre-order(\$left),
local:pre-order(\$right)
)
) else (
\$tree/@val/fn:string()
)
return \$seq
};

declare function local:has-children(
\$node as node()*
) {
if (fn:count(\$node) gt 1) then (
fn:error(xs:QName(“ERROR”), “not a single node”)
) else (
if (\$node/node()) then (
fn:true()
) else (
fn:false()
)
)
};

let \$tree :=

return

{local:pre-order(\$tree)}
{local:post-order(\$tree)}

7. Marc Young said

My formatting got fubar apparently…. the functions are correct though

8. treeowl said

Congrats to Josef Svenningsson for finding an O(n) prebuild! The tree it produces doesn’t quite match the specified format, but it’s easy to write a function to convert it to a tree that stores things in its leaves, so that’s no big deal. postbuild seems inherently harder. I have the feeling it should be possible to write something very similar to prebuild to apply to a reversed postorder traversal, but I haven’t managed to make it work out quite right yet. I will give it another go later.

9. […] This submission to Programming Praxis gives an O(n) function that “undoes” a preorder traversal of a binary search tree, converting a list back into a tree. Supplying the missing data declaration: […]