algorithm to find left view and right view of binary tree in java

This algorithm is an extension of breadth first search. In BFS we just print level wise nodes whereas here while printing level wise nodes we add temporary node in between every level (we can call it is Level_Separator (LS)). Once this is structure is formed then the node before the marker belongs to the right view and the node after the marker belongs to the left view.

What is left view? Nodes which are visible when looking from left. What is right view? Nodes which are visible when looking from right. In a tree like below one

            20
    15              25
      18      22      

Left view nodes are ‘20,15,18’ and right view nodes are ‘20,25,22’

Lets look at an example to understand this algorithm:

            20
    15              25
12      18      22      28

BFS puts this tree in a queue such that all level wise nodes are placed next to each other like this

BFS Queue
	--------------------------------
	20 | 15 | 25 | 12 | 18 | 22 | 28
	--------------------------------

Now add level separator in the above queue

Queue Modified for Left/Right Views
	-----------------------------------------------
	20 | LS | 15 | 25 | LS | 12 | 18 | 22 | 28 | LS
	-----------------------------------------------

Let us take nodes before Level_Separator (LS), i.e, 25,28 and after LS i.e, 15,12 excluding root node.

Now add root node to both before/after LS elements; 20,25,28 and 20,15,12 which are nothing but right view and left view of the binary tree.

	/**
	 * Get left and right view of the binary tree
	 * @param node root of the tree
	 */
	public void printLeftNRightViewOfBT(BinaryNode node) {
		Queue<BinaryNode> queue = new LinkedList<BinaryNode>();
		BinaryNode LS = new BinaryNode(null);
		queue.add(node);
		queue.add(LS);

		// left view and right view arrays
		List<BinaryNode> leftView = new ArrayList<BinaryNode>();
		List<BinaryNode> rightView = new ArrayList<BinaryNode>();
		leftView.add(node);

		while (!queue.isEmpty()) {
			BinaryNode currentNode = queue.remove();
			if (!queue.isEmpty() && currentNode.equals(LS)) {
				leftView.add(queue.peek());
				queue.add(LS);
			}
			if (!queue.isEmpty() && queue.peek().equals(LS)) {
				rightView.add(currentNode);
			}
			if (currentNode.left != null) {
				queue.add(currentNode.left);
			}
			if (currentNode.right != null) {
				queue.add(currentNode.right);
			}
		}

		// print views
		printViews(leftView);
		printViews(rightView);
	}

Output is:

Printing left View: 
50 17 9 14 12 
Printing right view: 
50 76 54 72 67