/**
 * 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) {
        Queue<TreeNode> que = new LinkedList<>();
        List<Double> ans = new ArrayList<>();
        
        if(root != null) {
            que.offer(root);
        }
        
        double len;
        double sum;
        while(!que.isEmpty()) {
            len = que.size();
            sum = 0;
            
            for(int i = 0; i < len; i++) {
                root = que.poll();
                sum += root.val;
                
                if(root.right != null) {
                    que.offer(root.right);
                }
                
                if(root.left != null) {
                    que.offer(root.left);
                }
            }
            
            ans.add(sum / len);
        }
        
        return ans;
    }
}