Tree traversal in smalltalk

Author: Neverik

contents/tree_traversal/code/smalltalk/tree_traversal.st

@@ -0,0 +1,101 @@
+Object subclass: #Node
+	instanceVariableNames: 'children data'
+	classVariableNames: ''
+	package: ''
+
+Node>>children
+	"Children getter."
+	^ children
+
+Node>>children: newChildren
+	"Children setter."
+	children := newChildren.
+
+Node>>data
+	"Data getter"
+	^ data
+
+Node>>data: newData
+	"Data setter"
+	data := newData.
+
+Node>>dfsRecursive
+	"Recursive depth first search."
+	data isNil ifFalse: [ 
+			Transcript show: data.
+			Transcript cr.
+		 ].
+	children collect: [ :each | each dfsRecursive].
+
+Node>>dfsRecursivePostorder
+	"Recursive depth first search (post-order)."
+	children collect: [ :each | each dfsRecursivePostorder ].
+	data isNil ifTrue: [ ^ self ].
+	Transcript show: data.
+	Transcript cr.
+
+Node>>dfsInOrderBinaryTree
+	"Recursive depth first search on a binary tree in order."
+	children size = 2 ifTrue: [ 
+			(children at: 1) dfsInOrderBinaryTree.
+        	Transcript show: data.
+			Transcript cr.
+			(children at: 2) dfsInOrderBinaryTree.
+			^self.
+		 ].
+	children size = 1 ifTrue: [ 
+			(children at: 1) dfsInOrderBinaryTree.
+        	Transcript show: data.
+			Transcript cr.
+			^self.
+		 ].
+	children size = 0 ifTrue: [ 
+        	Transcript show: data.
+			Transcript cr.
+			^self.
+		 ].
+	Transcript show: 'This is not a binary tree!'.
+	Transcript cr.
+	children length 
+
+Node>>dfsStack
+    "Depth-first search with a stack."
+    | stack top |
+    stack := Stack new.
+    stack push: self.
+	 [stack size > 0] whileTrue: [
+        top := stack pop.
+        Transcript show: top data.
+        Transcript cr.
+        top children reverseDo: [ :child |
+            stack push: child
+        ].
+    ].
+
+Node>>bfs
+	"A breadth-first tree search using queues."
+	| queue current |
+	queue := LinkedList with: self.
+  	[ queue size > 0 ] whileTrue: [
+    	current := queue first.
+		queue removeFirst.
+    	Transcript show: current.
+		Transcript cr.
+    	current children collect: [ :child |
+			 queue addLast: child
+		].
+  	]
+
+| test |
+test := Node new: 1 children: { Node new: 2.
+                                Node new: 3 children: { Node new: 4.
+                                                        Node new: 5. } }.
+test dfsRecursive.
+Transcript cr.
+test dfsRecursivePostorder.
+Transcript cr.
+test dfsInOrderBinaryTree.
+Transcript cr.
+test dfsStack.
+Transcript cr.
+test bfs.

contents/tree_traversal/tree_traversal.md

@@ -32,6 +32,8 @@ This has not been implemented in your chosen language, so here is the Julia code
 [import:3-27, lang:"php"](code/php/tree_traversal.php)
 {% sample lang="crystal" %}
 [import:1-5, lang:"crystal"](code/crystal/tree-traversal.cr)
+{% sample lang="st" %}
+[import:1-20, lang:"st"](code/smalltalk/tree_traversal.st)
 {% endmethod %}
 
 Because of this, the most straightforward way to traverse the tree might be recursive. This naturally leads us to the Depth-First Search (DFS) method:
@@ -66,6 +68,8 @@ Because of this, the most straightforward way to traverse the tree might be recu
 [import:31-35, lang:"php"](code/php/tree_traversal.php)
 {% sample lang="crystal" %}
 [import:7-10, lang:"crystal"](code/crystal/tree-traversal.cr)
+{% sample lang="st" %}
+[import:22-28, lang:"st"](code/smalltalk/tree_traversal.st)
 {% endmethod %}
 
 At least to me, this makes a lot of sense. We fight recursion with recursion! First, we first output the node we are on and then we call `DFS_recursive(...)` on each of its children nodes. This method of tree traversal does what its name implies: it goes to the depths of the tree first before going through the rest of the branches. In this case, the ordering looks like:
@@ -108,6 +112,8 @@ Now, in this case the first element searched through is still the root of the tr
 [import:37-41, lang:"php"](code/php/tree_traversal.php)
 {% sample lang="crystal" %}
 [import:12-15, lang:"crystal"](code/crystal/tree-traversal.cr)
+{% sample lang="st" %}
+[import:30-35, lang:"st"](code/smalltalk/tree_traversal.st)
 {% endmethod %}
 
 <p>
@@ -145,6 +151,8 @@ In this case, the first node visited is at the bottom of the tree and moves up t
 [import:43-62, lang:"php"](code/php/tree_traversal.php)
 {% sample lang="crystal" %}
 [import:17-31, lang:"crystal"](code/crystal/tree-traversal.cr)
+{% sample lang="st" %}
+[import:37-59, lang:"st"](code/smalltalk/tree_traversal.st)
 {% endmethod %}
 
 <p>
@@ -192,6 +200,8 @@ In code, it looks like this:
 [import:64-73, lang:"php"](code/php/tree_traversal.php)
 {% sample lang="crystal" %}
 [import:33-41, lang:"crystal"](code/crystal/tree-traversal.cr)
+{% sample lang="st" %}
+[import:61-73, lang:"st"](code/smalltalk/tree_traversal.st)
 {% endmethod %}
 
 All this said, there are a few details about DFS that might not be idea, depending on the situation. For example, if we use DFS on an incredibly long tree, we will spend a lot of time going further and further down a single branch without searching the rest of the data structure. In addition, it is not the natural way humans would order a tree if asked to number all the nodes from top to bottom. I would argue a more natural traversal order would look something like this:
@@ -231,6 +241,8 @@ And this is exactly what Breadth-First Search (BFS) does! On top of that, it can
 [import:65-74, lang:"php"](code/php/tree_traversal.php)
 {% sample lang="crystal" %}
 [import:43-51, lang:"crystal"](code/crystal/tree-traversal.cr)
+{% sample lang="st" %}
+[import:75-87, lang:"st"](code/smalltalk/tree_traversal.st)
 {% endmethod %}
 
 ## Example Code
@@ -272,6 +284,8 @@ The code snippets were taken from this [Scratch project](https://scratch.mit.edu
 [import, lang:"php"](code/php/tree_traversal.php)
 {% sample lang="crystal" %}
 [import, lang:"crystal"](code/crystal/tree-traversal.cr)
+{% sample lang="st" %}
+[import, lang:"st"](code/smalltalk/tree_traversal.st)
 {% endmethod %}