Bottom view of BT

class Solution
{
    class Pair {
        public int index;
        public Node value;
        public Pair(int i, Node v) {
            this.index = i;
            this.value = v;
        }
    }
    public void levelOrder(Node root, HashMap<Integer, Integer> map) {
        Queue<Pair> queue = new LinkedList<>();
        queue.add(new Pair(0, root));
        queue.add(new Pair(0, null));
        int i;
        while (!queue.isEmpty()) {
            if (queue.peek().value == null) {
                queue.remove();
                if (queue.isEmpty()) break;
                queue.add(new Pair(0, null));
            }
            else {
                i = queue.peek().index;
                map.put(i, queue.peek().value.data);
                Pair temp = queue.remove();
                if (temp.value.left != null) queue.add(new Pair(i - 1, temp.value.left));
                if (temp.value.right != null) queue.add(new Pair(i + 1, temp.value.right));
            }
        }
    }
    public ArrayList <Integer> bottomView(Node root) {
        ArrayList<Integer> list = new ArrayList<>();
        HashMap<Integer, Integer> map = new HashMap<>();
        if (root != null)
            levelOrder(root, map);
        PriorityQueue<Integer> pq = new PriorityQueue<>(map.keySet());
        while(!pq.isEmpty()) {
            list.add(map.get(pq.remove()));
        }
        return list;
    }
}