Extract leaves of a binary tree in a doubly linked list in typescript

Ts program for Extract leaves of a binary tree in a doubly linked list. Here mentioned other language solution.

/* 
  TypeScript program for
  Extract leaves of a binary tree in a doubly linked list
*/
class LinkNode
{
	public data: number;
	public left: LinkNode;
	public right: LinkNode;
	constructor(data: number)
	{
		// Set node value
		this.data = data;
		this.left = null;
		this.right = null;
	}
}
class BinaryTree
{
	public root: LinkNode;
	public front: LinkNode;
	constructor()
	{
		// Set initial tree root to null
		this.root = null;
		// This is initial linked list node
		this.front = null;
	}
	// Recursive function
	// Display preorder view of binary tree
	public preOrder(node: LinkNode)
	{
		if (node != null)
		{
			// Print node value
			console.log("  " + node.data);
			this.preOrder(node.left);
			this.preOrder(node.right);
		}
	}
	// Display extract leaves in tree
	public printDoublyList()
	{
		if (this.front == null)
		{
			console.log("\n Empty Linked List ");
			return;
		}
		console.log("\n Doubly Linked list : ");
		// Start to front node
		var temp = this.front;
		// Iterate Linked list elements
		while (temp != null)
		{
			// Display node value
			console.log("  " + temp.data);
			// Visit to next node
			temp = temp.right;
		}
		console.log();
	}
	// Extract leaf nodes in given binary tree and
	// added into doubly linked list
	public LinkNode extractLeaves(node: LinkNode)
	{
		if (node != null)
		{
			if (node.left == null && node.right == null)
			{
				// Add node at front of linked list
				node.right = this.front;
				if (this.front != null)
				{
					// When this is not first node
					// Combine left direction
					this.front.left = node;
				}
				this.front = node;
				// Returns null value to the previous node child
				return null;
			}
			node.right = this.extractLeaves(node.right);
			node.left = this.extractLeaves(node.left);
			return node;
		}
		return null;
	}
	public static main()
	{
		// Make new Binary Tree
		var tree = new BinaryTree();
		/*
		    Construct Binary Tree
		    -----------------------
		           10
		          / \
		         /   \
		        6     8
		       / \   / \
		      2   3 7   5
		     /       \   \
		    9        -6   13
		*/
		// Add nodes
		tree.root = new LinkNode(10);
		tree.root.left = new LinkNode(6);
		tree.root.left.left = new LinkNode(2);
		tree.root.right = new LinkNode(8);
		tree.root.right.right = new LinkNode(5);
		tree.root.right.left = new LinkNode(7);
		tree.root.right.left.right = new LinkNode(-6);
		tree.root.left.right = new LinkNode(3);
		tree.root.left.left.left = new LinkNode(9);
		tree.root.right.right.right = new LinkNode(13);
		// Display tree elements
		console.log("\n Preorder : ");
		tree.preOrder(tree.root);
		tree.extractLeaves(tree.root);
		// After transform
		console.log("\n After extract leaf node");
		console.log("\n Preorder : ");
		tree.preOrder(tree.root);
		tree.printDoublyList();
	}
}
BinaryTree.main();
/*
 file : code.ts
 tsc --target es6 code.ts
 node code.js
 */

Output

 Preorder :
  10
  6
  2
  9
  3
  8
  7
  -6
  5
  13

 After extract leaf node

 Preorder :
  10
  6
  2
  8
  7
  5

 Doubly Linked list :
  9
  3
  -6
  13