5: Implement post-order traversal algorithm (#16)

src/commonMain/kotlin/com.github.adriankuta/TreeNode.kt

@@ -1,5 +1,9 @@
 package com.github.adriankuta
 
+import com.github.adriankuta.iterators.PostOrderTreeIterator
+import com.github.adriankuta.iterators.PreOrderTreeIterator
+import com.github.adriankuta.iterators.TreeNodeIterators
+import com.github.adriankuta.iterators.TreeNodeIterators.*
 import kotlin.jvm.JvmSynthetic
 
 open class TreeNode<T>(val value: T) : Iterable<TreeNode<T>>, ChildDeclarationInterface<T> {
@@ -18,6 +22,8 @@ open class TreeNode<T>(val value: T) : Iterable<TreeNode<T>>, ChildDeclarationIn
     val children: List<TreeNode<T>>
         get() = _children
 
+    var defaultIterator: TreeNodeIterators = PreOrder
+
     /**
      * Add new child to current node or root.
      *
@@ -139,5 +145,8 @@ open class TreeNode<T>(val value: T) : Iterable<TreeNode<T>>, ChildDeclarationIn
      * Output: 1 2 5 10 6 11 12 13 3 4 7 8 9
      * ```
      */
-    override fun iterator(): Iterator<TreeNode<T>> = PreOrderTreeIterator(this)
+    override fun iterator(): Iterator<TreeNode<T>> = when (defaultIterator) {
+        PreOrder -> PreOrderTreeIterator(this)
+        PostOrder -> PostOrderTreeIterator(this)
+    }
 }

src/commonMain/kotlin/com.github.adriankuta/TreeNodeExt.kt

@@ -1,5 +1,6 @@
 package com.github.adriankuta
 
+import com.github.adriankuta.iterators.TreeNodeIterators
 import kotlin.jvm.JvmSynthetic
 
 typealias ChildDeclaration<T> = ChildDeclarationInterface<T>.() -> Unit
@@ -13,8 +14,13 @@ typealias ChildDeclaration<T> = ChildDeclarationInterface<T>.() -> Unit
  * @see [ChildDeclarationInterface.child]
  */
 @JvmSynthetic
-inline fun<reified T> tree(root: T, childDeclaration: ChildDeclaration<T>): TreeNode<T> {
+inline fun <reified T> tree(
+    root: T,
+    defaultIterator: TreeNodeIterators = TreeNodeIterators.PreOrder,
+    childDeclaration: ChildDeclaration<T>
+): TreeNode<T> {
     val treeNode = TreeNode(root)
+    treeNode.defaultIterator = defaultIterator
     treeNode.childDeclaration()
     return treeNode
 }

src/commonMain/kotlin/com.github.adriankuta/iterators/PostOrderTreeIterator.kt

@@ -0,0 +1,50 @@
+package com.github.adriankuta.iterators
+
+import com.github.adriankuta.TreeNode
+
+/**
+ * Tree is iterated by using `Post-order Traversal Algorithm"
+ *  1. Check if the current node is empty or null.
+ *  2. Display the data part of the root (or current node).
+ *  3. Traverse the left subtree by recursively calling the pre-order function.
+ *  4. Traverse the right subtree by recursively calling the pre-order function.
+ * ```
+ * E.g.
+ *                    1
+ *                  / | \
+ *                 /  |   \
+ *               2    3     4
+ *              / \       / | \
+ *             5    6    7  8  9
+ *            /   / | \
+ *           10  11 12 13
+ *
+ * Output: 1 2 5 10 6 11 12 13 3 4 7 8 9
+ * ```
+ */
+class PostOrderTreeIterator<T>(root: TreeNode<T>) : Iterator<TreeNode<T>> {
+
+    private val stack = ArrayDeque<TreeNode<T>>()
+
+    init {
+        stack.addAll(getChildrenStack(root))
+    }
+
+    override fun hasNext(): Boolean = stack.isNotEmpty()
+
+    override fun next(): TreeNode<T> {
+        return stack.removeFirst()
+    }
+
+    private fun getChildrenStack(node: TreeNode<T>): ArrayDeque<TreeNode<T>> {
+        val stack = ArrayDeque<TreeNode<T>>()
+        if(node.children.isEmpty()) {
+            return ArrayDeque(listOf(node))
+        }
+        node.children.forEach {
+            stack.addAll(getChildrenStack(it))
+        }
+        stack.addLast(node)
+        return stack
+    }
+}

src/commonMain/kotlin/com.github.adriankuta/iterators/PreOrderTreeIterator.kt

@@ -1,4 +1,6 @@
-package com.github.adriankuta
+package com.github.adriankuta.iterators
+
+import com.github.adriankuta.TreeNode
 
 /**
  * Tree is iterated by using `Pre-order Traversal Algorithm"
@@ -31,10 +33,12 @@ class PreOrderTreeIterator<T>(root: TreeNode<T>) : Iterator<TreeNode<T>> {
     override fun hasNext(): Boolean = stack.isNotEmpty()
 
     override fun next(): TreeNode<T> {
+        println(stack)
         val node = stack.removeLast()
         node.children
             .asReversed()
             .forEach { stack.addLast(it) }
+        println(stack)
         return node
     }
 }

src/commonMain/kotlin/com.github.adriankuta/iterators/TreeNodeIterators.kt

@@ -0,0 +1,5 @@
+package com.github.adriankuta.iterators
+
+enum class TreeNodeIterators {
+    PreOrder, PostOrder
+}