题目描述：

标签：树  

给定一个非空二叉树, 返回一个由每层节点平均值组成的数组。

代码：

思路分析：思路同二叉树的层序遍历。只不过需要每层对所有节点的值求和，然后每层遍历结束后将该层的平均值加入链表。

/**
 * 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) {
        List<Double> list = new ArrayList<Double>();
        if(root == null){
            return list;
        }

        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        while(!queue.isEmpty()){
            int currentLevelSize = queue.size();
            Double sum = 0.0;
            for(int i = 1;i <= currentLevelSize;i++){
                TreeNode node = queue.poll();
                sum = sum + node.val;
                if(node.left != null){
                    queue.offer(node.left);
                }
                if(node.right != null){
                    queue.offer(node.right);
                }
            }
            list.add(sum/currentLevelSize);
        }
        return list;
    }
}