Average of Levels in Binary Tree

Given a non-empty binary tree, return the average value of the nodes on each level in the form of an array.

Example 1:

Input:
    3
   / 
  9  20
    /  
   15   7
Output: [3, 14.5, 11]
Explanation:
The average value of nodes on level 0 is 3,  on level 1 is 14.5, and on level 2 is 11. Hence return [3, 14.5, 11].

还是应用广度优先遍历（BFS），然后计算每一层的和就行

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