Bottom View of Binary Tree

class Solution {
    public ArrayList<Integer> bottomView(Node root) {
        ArrayList<Integer> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        
        Map<Integer, Integer> map = new TreeMap<>();
        Queue<Node> queue = new LinkedList<>();
        queue.add(root);
        root.hd = 0;
        
        while (!queue.isEmpty()) {
            Node temp = queue.poll();
            map.put(temp.hd, temp.data);
            
            if (temp.left != null) {
                temp.left.hd = temp.hd - 1;
                queue.add(temp.left);
            }
            
            if (temp.right != null) {
                temp.right.hd = temp.hd + 1;
                queue.add(temp.right);
            }
        }
        
        for (int value : map.values()) {
            result.add(value);
        }
        
        return result;
    }
}