Generic Recursive Postorder Binary Tree Traversal in Java

A binary tree is a tree data structure in which each node has at most two children, which are referred to as the left child and the right child.

Postorder traversal allows all the nodes of the binary tree to be visited by applying a recursive algorithm.

This can be summed up as

1. Traverse left subtree
2. Traverse right subtree
3. Visit root node (output this)

Recursive postorder binary tree traversal algorithm can use a generic node class.

Postorder Traversal Sequence: 4 - 5 - 2 - 6 - 3 - 1

package interviewquestions;


class Node<T>
{
   private T data;
   private Node<T> left;
   private Node<T> right;
 
   Node( T pData, Node<T> pLeft, Node<T> pRight )
   {
     data = pData;
     left = pLeft;
     right = pRight;
   }
 
   public void setLeftChild( Node<T> pLeft ) {  left = pLeft; }  
   public void setRightChild( Node<T> pRight ) { right = pRight; } 
 
   public Node<T> getLeftChild() { return left; }
   public Node<T> getRightChild() { return right; }  
   public T getData() { return data; }

}

class BinaryTree<T>
{
   Node<T> root;
 
   public BinaryTree(){  root = null; } 
 
   public void setRootNode( Node<T> pRoot ) {  root = pRoot; }
 
   public void postOrder()
   {
     postOrder( root );   
   }
 
   private void postOrder( Node<T> pNode )
   {
     if( pNode == null )
       return;   

     postOrder( pNode.getLeftChild() );
     postOrder( pNode.getRightChild() );

     System.out.println( pNode.getData() );  

   }
  
}

public class PostOrderTraversal {
 
 public static void main(String[] args) {
  
    Node<Integer> node1 = new Node<Integer>( 1, null, null );
    Node<Integer> node2 = new Node<Integer>( 2, null, null );
    Node<Integer> node3 = new Node<Integer>( 3, null, null );
    Node<Integer> node4 = new Node<Integer>( 4, null, null );
    Node<Integer> node5 = new Node<Integer>( 5, null, null );
    Node<Integer> node6 = new Node<Integer>( 6, null, null );
  
    node1.setLeftChild(node2);
    node1.setRightChild(node3);
  
    node2.setLeftChild(node4);
    node2.setRightChild(node5);
  
    node3.setRightChild(node6);
  
    BinaryTree<Integer> binaryTree = new BinaryTree<Integer>();
    binaryTree.setRootNode(node1);
  
    binaryTree.postOrder();  
 }

}

Create a PostOrderTraversal.java file in your workspace.

When the main method inside the PostOrderTraversal class executed it is going to print :

4
5
2
6
3
1