1、前言

2、思路

简单的层次遍历。

3、代码

/**
 * 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> res = new ArrayList<>();

        if(root == null){
            return res;
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);

        while(!queue.isEmpty()){
            int size = queue.size();
            double nums = 0.0d;
            for(int i = 0; i < size; i++){
                TreeNode cur = queue.poll();
                nums += cur.val;

                if(cur.left != null){
                    queue.offer(cur.left);
                }
                if(cur.right != null){
                    queue.offer(cur.right);
                }
            }
            res.add(nums / size);
        }

        return res;
    }
}