## Binary Tree Traversal

### October 18, 2013

The pre-order and post-order functions are simple recursive functions on the structure of the tree; notice that we are careful to avoid nulls in the tree structure:

```(define (preorder t)   (if (null? t)       (list)       (append (list (car t))               (if (pair? (cdr t))                   (preorder (cadr t))                   (list))               (if (and (pair? (cdr t))                        (pair? (cddr t)))                   (preorder (caddr t))                   (list)))))```

```(define (postorder t)   (if (null? t)       (list)       (append (if (pair? (cdr t))                   (postorder (cadr t))                   (list))               (if (and (pair? (cdr t))                        (pair? (cddr t)))                   (postorder (caddr t))                   (list))               (list (car t)))))```

And here are the functions applied to the sample tree shown on the previous page:

```> (define t '(8 (3 (1) (6 (4) (7))) (10 () (14 (13) ())))) > (preorder t) (8 3 1 6 4 7 10 14 13) > (postorder t) (1 4 7 6 3 13 14 10 8)```

Reconstructing the tree is the opposite operation of traversing the tree. The left-most element of the pre-order sequence is the root of the tree. Nodes less than the root are in the left child of the tree, and nodes greater than the root are in the right child of the tree, so the solution makes the first node in the pre-order sequence the root of the tree and calls itself recursively on the the lesser portion of the sequence and the greater portion of the sequence. Reconstructing a tree based on the post-order sequence is the opposite, with the root of the tree at the last element of the list:

```(define (prebuild xs)   (cond ((null? xs) (list))       ((null? (cdr xs)) (list (car xs)))         (else (call-with-values           (lambda ()             (split-while               (lambda (x) (< x (car xs))) (cdr xs)))           (lambda (lo hi)               (list (car xs) (prebuild lo) (prebuild hi)))))))```

```(define (postbuild xs)   (cond ((null? xs) (list))         ((null? (cdr xs)) (list (car xs)))         (else (call-with-values           (lambda ()             (split-while               (lambda (x) (< x (last xs))) (but-last xs)))             (lambda (lo hi)               (list (last xs) (postbuild lo) (postbuild hi)))))))```

We use the convenience functions `last` and `but-last`:

`(define (last xs) (car (reverse xs)))`

`(define (but-last xs) (reverse (cdr (reverse xs))))`

And here are some more examples:

```> (prebuild (preorder t)) (8 (3 (1) (6 (4) (7))) (10 () (14 (13) ()))) > (postbuild (postorder t)) (8 (3 (1) (6 (4) (7))) (10 () (14 (13) ())))```

We used `split-while` from the Standard Prelude. You can run the program at http://programmingpraxis.codepad.org/s86rBxGF.

Pages: 1 2

### 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),o(b,c),o(b,c),d||c.push(b)):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),o(b,c,d),o(b,c,d),d||c.push(b)):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()
let \$right := \$tree/node()
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()
let \$right := \$tree/node()
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: […]