void top_view(Node root)
{
    Node curr = root;
    Stack<Node> stack = new Stack<Node>();
    while(curr != null) {
        stack.push(curr);
        curr = curr.left;
    }
    
    while(!stack.isEmpty()){
        Node node = stack.pop();
        System.out.print(node.data + " ");
    }
    
    curr = root.right;
    while(curr != null){
        System.out.print(curr.data + " ");
        curr = curr.right;
    }
}