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POPre-order/Post-order iterative traversal of n-ary tree using Iterator pattern
 

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  1. POPre-order/Post-order iterative traversal of n-ary tree using Iterator pattern
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    <p>I have implemented a Generic (n-ary) tree in Java as given <a href="http://vivin.net/2010/01/30/generic-n-ary-tree-in-java/" rel="nofollow noreferrer">here</a> and by referencing the source given on the <a href="https://github.com/vivin/GenericTree" rel="nofollow noreferrer">GitHub</a> repository of the author <a href="http://vivin.net/2010/01/30/generic-n-ary-tree-in-java/" rel="nofollow noreferrer">1</a>. I want to implement a pre-order and post-order traversal of the n-ary tree using the Iterator in java. Thus, the methods hasNext() will return a true whenever there is a node and the method next() will return the node which would be present next in the pre/post-order traversal.</p> <p>I am trying to follow the pseudo-codes given in this <a href="https://stackoverflow.com/questions/5987867/traversing-a-n-ary-tree-without-using-recurrsion">question</a> but I am not able to plug it in the next method of Iterator which I've written below</p> <pre><code>public class DepthFirstIterator&lt;T&gt; implements Iterator&lt;TreeNode&lt;T&gt;&gt; { private Stack&lt;TreeNode&lt;T&gt;&gt; dfsStack; private Tree&lt;T&gt; tree; private TreeNode&lt;T&gt; start; public DepthFirstIterator(Tree&lt;T&gt; tree) { this.tree = tree; this.dfsStack = new Stack&lt;TreeNode&lt;T&gt;&gt;(); if (!this.tree.isEmpty()) this.dfsStack.push(this.tree.getRoot()); } public DepthFirstIterator(Tree&lt;T&gt; tree, TreeNode&lt;T&gt; startNode) { this.tree = tree; this.dfsStack = new Stack&lt;TreeNode&lt;T&gt;&gt;(); if (startNode != null) this.dfsStack.push(startNode); } public boolean hasNext() { return (!this.dfsStack.isEmpty()); } public TreeNode&lt;T&gt; next() { // Iterative code to obtain pre/post-order traversal } public void remove() { // Do nothing } </code></pre> <p>Tree Class:</p> <pre><code>public class Tree&lt;T&gt; { private TreeNode&lt;T&gt; root; public TreeNode&lt;T&gt; getRoot() { return this.root; } public void setRoot(TreeNode&lt;T&gt; element) { this.root = element; } public boolean isEmpty() { return (this.root == null); } public int size() { if (isEmpty()) return 0; else return getNumberOfNodes(root) + 1; } private int getNumberOfNodes(TreeNode&lt;T&gt; node) { int num = 0; Stack&lt;TreeNode&lt;T&gt;&gt; nodeStack = new Stack&lt;TreeNode&lt;T&gt;&gt;(); nodeStack.push(node); while (!nodeStack.isEmpty()) { TreeNode&lt;T&gt; top = nodeStack.pop(); for (TreeNode&lt;T&gt; child : top.getChildren()) { num++; nodeStack.push(child); } } return num; } } </code></pre> <p>TreeNode class:</p> <pre><code>public class TreeNode&lt;T&gt; { private T data; private List&lt;TreeNode&lt;T&gt;&gt; children; private TreeNode&lt;T&gt; parent; public TreeNode() { super(); children = new ArrayList&lt;TreeNode&lt;T&gt;&gt;(); parent = null; } public TreeNode(T data) { this(); setData(data); } public void setData(T data) { this.data = data; } public T getData() { return this.data; } public List&lt;TreeNode&lt;T&gt;&gt; getChildren() { return children; } public void setChildren(List&lt;TreeNode&lt;T&gt;&gt; children) { for (TreeNode&lt;T&gt; child : children) child.parent = this; this.children = children; } public void addChild(TreeNode&lt;T&gt; child) { child.parent = this; children.add(child); } public void insertChildAt(int index, TreeNode&lt;T&gt; child) throws IndexOutOfBoundsException { child.parent = this; children.add(index, child); } public TreeNode&lt;T&gt; getChildAt(int index) throws IndexOutOfBoundsException { return children.get(index); } public void removeChildAt(int index) throws IndexOutOfBoundsException { children.remove(index); } public void removeChildren() { children.clear(); } public int getNumberOfChildren() { return children.size(); } public String toString() { return getData().toString(); } public boolean hasChildren() { return (getChildren().size() &gt; 0); } public TreeNode&lt;T&gt; getParent() { return this.parent; } } </code></pre> <p>I know that using as-many for blocks as the depth of the tree goes is totally wrong, but the stack logic was not being intuitive to me. If somebody,could please guide me I would really appreciate it.</p> <p>Thanks, Chaitanya</p>
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    1. USchaitanya
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    1. COI don't understand your question. You've already (or copied) a `next()` method. What is your problem with it?
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    2. COThe method is highly incorrect. I am using 2 for loops which will only work for a depth of 3 in the n-ary tree. Thus, it is not at all generic.
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    3. COYou have to use recursion. I'm sure you will find similar examples if you google for "recursive tree traversal".
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