CC-13 : Traverse Binary Tree via Post-order using recursion.

Description:

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

Test cases and expected outputs:

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

Pseudocode:

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

Method postOrderRecursive(parentNode):

If parentNode is null, return.

Call postOrderRecursive() with parent node’s left child as parameter.

Call postOrderRecursive() with parent node’s right child as parameter.

Add parent node’s node data to Post-order traversal ArrayList.

Code:

public class BinTreePostOrderRecursive {

private ArrayList<Integer> postOrder=new ArrayList<Integer>();
public ArrayList<Integer> getPostOrder() {
	return postOrder;
}
public void setPostOrder(ArrayList<Integer> postOrder) {
	this.postOrder = postOrder;
}

public void postOrderRecursive(BinTreeNode parent) throws Exception{
	if (parent==null) {return;}
	postOrderRecursive(parent.getLeftChild());
	postOrderRecursive(parent.getRightChild());
	postOrder.add(parent.getNodeData());
}
}

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;

public class BinTreePostOrderRecursive {

private ArrayList<Integer> postOrder=new ArrayList<Integer>();
public ArrayList<Integer> getPostOrder() {
	return postOrder;
}
public void setPostOrder(ArrayList<Integer> postOrder) {
	this.postOrder = postOrder;
}

public void postOrderRecursive(BinTreeNode parent) throws Exception{
	if (parent==null) {return;}
	postOrderRecursive(parent.getLeftChild());
	postOrderRecursive(parent.getRightChild());
	postOrder.add(parent.getNodeData());
}

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 {
	BinTreePostOrderRecursive po=new BinTreePostOrderRecursive();
	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));
	
	po.postOrderRecursive(root);
	printArrayListSummary(po.getPostOrder(), "PostOrder 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;
	}

}
	
}