Generic Recursive Preorder 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.

Preorder traversal allows all the nodes of the binary tree to be visited by starting from the root node.

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

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

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 preOrder()
   {
     preOrder( root );   
   }
 
   private void preOrder( Node<T> pNode )
   {
     if( pNode == null )
       return;   
     System.out.println( pNode.getData() );  
  
     preOrder( pNode.getLeftChild() );
     preOrder( pNode.getRightChild() );
   }
  
}

public class PreOrderTraversal {
 
 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.preOrder();  
 }

}

Create a PreOrderTraversal.java file in your workspace.

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

1
2
4
5
3
6