637. 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].
题目含义：求二叉树中每一层的平均值

    public List<Double> averageOfLevels(TreeNode root) {
        List<Double> result = new ArrayList<>();
        Queue<TreeNode> stack = new LinkedList<>();
        stack.add(root);
        while (!stack.isEmpty())
        {
            int n = stack.size();
            Double sum = 0.0;
            for (int i=0;i<n;i++)
            {
                TreeNode node = stack.poll();
                sum += node.val;
                if (node.left!= null) stack.add(node.left);
                if (node.right!=null) stack.add(node.right);
            }
            result.add(sum/n);
        }
        return result;        
    }