class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        if (root == null) return Collections.emptyList();
        LinkedList<TreeNode> q = new LinkedList<>();
        List<Integer> res = new ArrayList<>();
        q.offer(root);
        while (!q.isEmpty()) {
            int size = q.size();
            Integer lastNode = null;
            for (int i = 0; i < size; i++) {
                TreeNode t = q.poll();
                lastNode = t.val;
                if (t.left != null) q.offer(t.left);
                if (t.right != null) q.offer(t.right);
            }
            res.add(lastNode);
        }
        return res;
    }
}