TreeTopView.java

   //new class to store level of each node along with the node 
   static class QueueNode{
        Node node;
        int level;
        public QueueNode(Node node, int level){
            this.node = node;
            this.level = level;
        }
    }


    public static void topView(Node root) {
        //took a queue - similar to BFS approach
        Queue<QueueNode> queue = new LinkedList<QueueNode>();
       
        //Taking a TreeMap<first, second>
        //first - stores the level of the node
        //second - stores the node.data
        TreeMap<Integer, Integer> map = new TreeMap<Integer, Integer>();
        //why TreeMap? Because I want the nodes to be sorted from leftmost to rightmost
        
        //start (since root, level = 0)
        queue.add(new QueueNode(root, 0));
        
        while(!queue.isEmpty()){
            //remove element from queue
            QueueNode r = queue.poll();
            //if the level of this node has never been reached before -> visible in top view
            if(!map.containsKey(r.level)){
                map.put(r.level, r.node.data);
            }
            
            //add node's descendants
            //all left child node levels = node.level - 1
            //all right child node levels = node.level + 1
            if(r.node.left != null){
                queue.add(new QueueNode(r.node.left, r.level - 1));
            }
            if(r.node.right != null){
                queue.add(new QueueNode(r.node.right, r.level + 1));
            }
        }
        //since already sorted (TreeMap, remember?), print from left to right
        for (Integer value : map.values()) {
            System.out.print(value + " ");
        }
    }