TopViewBT.java

public class Main {

    public static Map<Integer, Integer> ht = new HashMap<>();;

    public static void topView(Node root, int level) {
        if (root == null)
            return;

        Queue<QueuePack> queue = new LinkedList<>();
        queue.add(new QueuePack(level, root));

        while (!queue.isEmpty()) {
            QueuePack q = queue.remove();
            Node tnode = q.tnode;
            int lvl = q.level;

            // check if this is the first node you are visiting at the level
            if (!ht.containsKey(lvl))  {
                // print it, its the first element at his level
                System.out.print(tnode.data + "   ");
                ht.put(lvl, tnode.data);
            }

            // add the left and right children of visiting nodes to the queue
            if (tnode.left != null) {
                queue.add(new QueuePack(lvl - 1, tnode.left));
            }
            if (tnode.right != null) {
                queue.add(new QueuePack(lvl + 1, tnode.right));
            }
        }

    }

    public static void main(String args[]) {
        Node root = new Node(1);
        root.left = new Node(2);
        root.right = new Node(3);
        root.left.left = new Node(4);
        root.left.left.left = new Node(8);
        root.left.left.right = new Node(9);
        root.left.right = new Node(5);
        root.right.left = new Node(6);
        root.right.right = new Node(7);

        topView(root, 0);
    }
}

class Node {
    int data;
    Node left;
    Node right;

    public Node(int data) {
        this.data = data;
        left = null;
        right = null;
    }
}

// this class' represents the items stored in queue
class QueuePack {
    int level;
    Node tnode;

    public QueuePack(int level, Node tnode) {
        this.level = level;
        this.tnode = tnode;
    }
}