Average of Levels in Binary Tree.java

Basic question about Level order traversal using BFS.
You can see the BFS Template here:
https://github.com/cherryljr/LintCode/blob/master/BFS%20Template.java

/*
Given a non-empty binary tree, return the average value of the nodes on each level in the form of an array.

Example 1:
Input:
    3
   / \
  9  20
    /  \
   15   7
Output: [3, 14.5, 11]

Explanation:
The average value of nodes on level 0 is 3,  on level 1 is 14.5, and on level 2 is 11. Hence return [3, 14.5, 11].

Note:
The range of node's value is in the range of 32-bit signed integer.
*/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Double> averageOfLevels(TreeNode root) {
        List<Double> rst = new ArrayList<>();
        if (root == null)  {
            return rst;
        }
        
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            double sum = 0;
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                sum += node.val;
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            rst.add(sum / size);
        }
        
        return rst;
    }
}