Tree Traversal : Inorder Preorder Postorder in Java

class Node
{
  int key;
  Node left, right;
  public Node (int item)
  {
    key = item;
    left = right = null;
  }
}

class BinaryTree
{
// Root of Binary Tree
  Node root;

    BinaryTree ()
  {
    root = null;
  }

  void postorder (Node ptr)
  {
    if (ptr == null)
      return;

// first recur on left subtree
    postorder (ptr.left);

// then recur on right subtree
    postorder (ptr.right);

// now deal with the node
    System.out.print (ptr.key + " ");
  }

/* Given a binary tree, print its nodes in inorder*/
  void inorder (Node ptr)
  {
    if (ptr == null)
      return;

/* first recur on left child */
    inorder (ptr.left);

/* then print the data of node */
    System.out.print (ptr.key + " ");

/* now recur on right child */
    inorder (ptr.right);
  }

  void preorder (Node ptr)
  {
    if (ptr == null)
      return;

/* first print data of node */
    System.out.print (ptr.key + " ");

/* then recur on left subtree */
    preorder (ptr.left);

/* now recur on right subtree */
    preorder (ptr.right);
  }
}

public class Main
{
  public static void main (String[]args)
  {
    BinaryTree tree = new BinaryTree ();
      tree.root = new Node (1);
      tree.root.left = new Node (2);
      tree.root.right = new Node (3);
      tree.root.left.left = new Node (4);
      tree.root.left.right = new Node (5);

      System.out.println ("Preorder traversal");
      tree.preorder (tree.root);

      System.out.println ("\nInorder traversal");
      tree.inorder (tree.root);

      System.out.println ("\nPostorder traversal");
      tree.postorder (tree.root);
  }
}