Average of Levels in Binary Tree

public List averageOfLevels(TreeNode root) { 
    List result = new ArrayList<>(); 
    Queue q = new LinkedList<>();
    if(root == null) return result;
    q.add(root);
    while(!q.isEmpty()) {
        int n = q.size();
        double sum = 0.0;
        for(int i = 0; i < n; i++) {
            TreeNode node = q.poll();
            sum += node.val;
            if(node.left != null) q.offer(node.left);
            if(node.right != null) q.offer(node.right);
        }
        result.add(sum / n);
    }
    return result;
}