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].
題目含義：求二叉樹中每一層的平均值

 1     public List<Double> averageOfLevels(TreeNode root) {
 2         List<Double> result = new ArrayList<>();
 3         Queue<TreeNode> stack = new LinkedList<>();
 4         stack.add(root);
 5         while (!stack.isEmpty())
 6         {
 7             int n = stack.size();
 
 8             Double sum = 0.0;
 9             for (int i=0;i<n;i++)
10             {
11                 TreeNode node = stack.poll();
12                 sum += node.val;
13                 if (node.left!= null) stack.add(node.left);
14                 if (node.right!=null) stack.add(node.right);
15
 
             }
16             result.add(sum/n);
17         }
18         return result;        
19     }

637. Average of Levels in Binary Tree