/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Double> averageOfLevels(TreeNode root) {
        List<Double> list = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        
        if(root == null)
            return list;
        queue.add(root);
        
        while(!queue.isEmpty())
        {
            int queueSize = queue.size();
            double sum = 0.0;
            
            for(int i=0; i<queueSize; i++)
            {
                TreeNode node = queue.poll();
                sum += node.val;
                if(node.left != null)
                    queue.offer(node.left);
                if(node.right != null)
                    queue.offer(node.right);
            }
            list.add(sum/queueSize);
        }
        return list;
        
    }
}