Generic Recursive Inorder 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.

Inorder 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. Visit root node (output this)
3. Traverse right subtree

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

                                                 Inorder Traversal Sequence: 4 - 2 - 5 - 1 - 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 inOrder()
   {
     inOrder( root );   
   }
 
   private void inOrder( Node<T> pNode )
   {
     if( pNode == null )
       return;   

     inOrder( pNode.getLeftChild() );
     System.out.println( pNode.getData() );
     inOrder( pNode.getRightChild() );       

   }
  
}

public class InOrderTraversal {
 
 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.inOrder();  
 }

}

Create a InOrderTraversal.java file in your workspace.

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

4
2
5
1
3
6