public static void topView(Node root) {
      if (root == null) {
        return;
      }
      
      class Pair {
        Node node;
        int hd;
        
        Pair(Node n, int h) {
            node = n;
            hd = h;
        }
      }
      
      Map<Integer, Integer> topViewMap = new TreeMap<>();
      Queue<Pair> queue = new LinkedList<>();
      queue.add(new Pair(root, 0));
      
      while (!queue.isEmpty()) {
        Pair current = queue.poll();
        Node currentNode = current.node;
        int hd = current.hd;
        
        if (!topViewMap.containsKey(hd)) {
            topViewMap.put(hd, currentNode.data);
        }
        
        if (currentNode.left != null) {
            queue.add(new Pair(currentNode.left, hd - 1));
        }
        
        if (currentNode.right != null) {
            queue.add(new Pair(currentNode.right, hd + 1));
        }
      }
      
      for (int value : topViewMap.values()) {
        System.out.print(value + " ");
      }
    }