/*** Definition for a binary tree node.* public class TreeNode {*     int val;*     TreeNode left;*     TreeNode right;*     TreeNode() {}*     TreeNode(int val) { this.val = val; }*     TreeNode(int val, TreeNode left, TreeNode right) {*         this.val = val;*         this.left = left;*         this.right = right;*     }* }*/
class Solution {public List<Double> averageOfLevels(TreeNode root) {if(root==null){return null;}//返回结果List<Double> list = new ArrayList<>();//统计队列中每层的大小int size = 0;//计算每层的和double sum = 0;//队列--实现层序遍历Queue<TreeNode> q = new LinkedList<>();q.offer(root);while(!q.isEmpty()){//获取当前队列有多少个元素size = q.size();sum = 0;//依次取出每层元素for(int i=0;i<size;i++){TreeNode node = q.poll();sum += node.val;//添加左右结点if(node.left!=null) q.offer(node.left);if(node.right!=null) q.offer(node.right);}//添加结果集list.add(sum/size);}return list;}
}