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

// A class to store node and its horizontal distance (hd)
class QueueNode {
    Node node;
    int hd;

    QueueNode(Node node, int hd) {
        this.node = node;
        this.hd = hd;
    }
}

Queue<QueueNode> queue = new LinkedList<>();
Map<Integer, Integer> topViewMap = new TreeMap<>(); // TreeMap to sort by HD

queue.add(new QueueNode(root, 0));

while (!queue.isEmpty()) {
    QueueNode current = queue.poll();
    int hd = current.hd;
    Node node = current.node;

    // Only add to map if this HD hasn't been seen yet
    if (!topViewMap.containsKey(hd)) {
        topViewMap.put(hd, node.data);
    }

    // Enqueue left child with HD-1
    if (node.left != null) {
        queue.add(new QueueNode(node.left, hd - 1));
    }

    // Enqueue right child with HD+1
    if (node.right != null) {
        queue.add(new QueueNode(node.right, hd + 1));
    }
}

// Print the top view in left-to-right order (sorted HD)
for (Map.Entry<Integer, Integer> entry : topViewMap.entrySet()) {
    System.out.print(entry.getValue() + " ");
}

}