CC-10 : Traverse Binary Tree via Pre-order.

Description:

As part of Pre-order all nodes are iteratively/ recursively traversed in order of parent node, left node, right node. Given an input Binary Tree, traverse the tree in Pre-order.

Test cases and expected outputs:

Input Parameters	Expected outputs
	PreOrder traversal : 4, 5, 6, 33, 78, 7, 14, 8, 16, 18, 9

Pseudocode:

Initialize an ArrayList to hold the result of the Pre-order traversal.

Initialize a Stack to temporarily hold Tree Nodes that are being processed in LIFO order.

Add the root node of Binary Tree to the Stack.

Iterate through stack nodes using an while loop, while Stack size is not zero:

Extract the first node of stack and assign it to a variable named parentNode.

Add parentNode’s node data to Pre-order traversal ArrayList.

If the parentNode’s right childs not null, add it the right child node to the Stack.

If the parentNode’s left childs not null, add it the left child node to the Stack.

Return Pre-order traversal ArrayList.

Code:

public ArrayList<Integer> preOrderIterative(BinTreeNode root) throws Exception{
	if (root==null) {return null;}
	ArrayList<Integer> preOrder=new ArrayList<Integer>();
	Deque<BinTreeNode> stack=new LinkedList<BinTreeNode>();
	stack.offerFirst(root);
	BinTreeNode parent=null;
	while (stack.size()!=0) {
		parent=stack.pollFirst();
		preOrder.add(parent.getNodeData());
		if (parent.getRightChild()!=null) {
			stack.offerFirst(parent.getRightChild());
		}
		if (parent.getLeftChild()!=null) {
			stack.offerFirst(parent.getLeftChild());
		}		
	}
	return preOrder;
}

Click here to download and run code and test cases !

Below fully running code can be copied and run on Eclipse or other Java IDEs. Refer the classname in code below. If the class name below is "A", save the code below to a file named A.java before running it. Be sure to try your own test cases to enhance your understanding ! You can also tweak the code to optimize or add enhancements and custom features.

Note: for easy reference BinTreeNode is included as internal class in program below. Ideally it should be in a separate file.

import java.util.ArrayList;
import java.util.Deque;
import java.util.LinkedList;

public class BinTreePreOrderIterative {	

public ArrayList<Integer> preOrderIterative(BinTreeNode root) throws Exception{
	if (root==null) {return null;}
	ArrayList<Integer> preOrder=new ArrayList<Integer>();
	Deque<BinTreeNode> stack=new LinkedList<BinTreeNode>();
	stack.offerFirst(root);
	BinTreeNode parent=null;
	while (stack.size()!=0) {
		parent=stack.pollFirst();
		preOrder.add(parent.getNodeData());
		if (parent.getRightChild()!=null) {
			stack.offerFirst(parent.getRightChild());
		}
		if (parent.getLeftChild()!=null) {
			stack.offerFirst(parent.getLeftChild());
		}		
	}
	return preOrder;
}

public static void printArrayListSummary(ArrayList al, String label) throws Exception {
	if (al == null) { System.out.println("\n\n Input ArrayList was null !! \n"); return; }
	System.out.println();
	System.out.println("***********************************************************************");
	System.out.print(label+" : ");
	int arrayIndex=0;
	for (arrayIndex=0; arrayIndex < al.size(); arrayIndex++) {
			System.out.print(al.get(arrayIndex));
			if (arrayIndex < al.size()-1) {System.out.print(",");}
	} 
	System.out.println();
	System.out.println("************************************************************************");
		System.out.println();
		Thread.sleep(2000);
		return;
}
 	
public static void main(String[] args) {
	/************************
 	Test cases given below:
	 **********************/
	ArrayList<Integer> retVal=null;
	try {
	BinTreePreOrderIterative po=new BinTreePreOrderIterative();
	BinTreeNode root=po.new BinTreeNode(4);
	//Left L1 Children
	root.setLeftChild(po.new BinTreeNode(5));
	root.setRightChild(po.new BinTreeNode(7));
	
	//Left L2 Children
	root.getLeftChild().setLeftChild(po.new BinTreeNode(6));
	root.getLeftChild().setRightChild(po.new BinTreeNode(78));
	
	//Left L3 Children
	root.getLeftChild().getLeftChild().setLeftChild(po.new BinTreeNode(33));
	
	//Right L2 Children
	root.getRightChild().setLeftChild(po.new BinTreeNode(14));
	
	
	//Right L3 Children
	root.getRightChild().getLeftChild().setLeftChild(po.new BinTreeNode(8));
	root.getRightChild().getLeftChild().setRightChild(po.new BinTreeNode(16));
	
	//Right L3 Children
	root.getRightChild().getLeftChild().getRightChild().setLeftChild(po.new BinTreeNode(18));
	
	//Right L4 Children
	root.getRightChild().getLeftChild().getRightChild().getLeftChild().setLeftChild(po.new BinTreeNode(9));	
	
	retVal=po.preOrderIterative(root);
	printArrayListSummary(retVal, "PreOrder traversal");	
	
	}catch (Exception exception) {
	System.out.print("Exception: "+ exception);
	exception.printStackTrace();
}
		
}

public class BinTreeNode {
	private BinTreeNode leftNode=null;
	private BinTreeNode rightNode=null;
	private int nodeData=0;
	
	public BinTreeNode(int data) {
		nodeData=data;
	}	
	public BinTreeNode getLeftChild() {
		return leftNode;
	}
	public void setLeftChild(BinTreeNode leftNode) {
		this.leftNode = leftNode;
	}
	public BinTreeNode getRightChild() {
		return rightNode;
	}
	public void setRightChild(BinTreeNode rightNode) {
		this.rightNode = rightNode;
	}
	public int getNodeData() {
		return nodeData;
	}
	public void setNodeData(int nodeData) {
		this.nodeData = nodeData;
	}

}
	
}