637. 二叉树的层平均值

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class Solution {
    public List<Double> averageOfLevels(TreeNode root) {

        List<Double> list = new LinkedList<>();
        Queue<TreeNode> queue = new LinkedList<>();

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

        queue.add(root);

        while (!queue.isEmpty()){

            int size = queue.size();
            double sum = 0.0;

            for (int i = 0; i < size; i++) {

                TreeNode temp = queue.poll();
                sum += temp.val;

                if (temp.left != null){
                    queue.add(temp.left);
                }

                if (temp.right != null) {
                    queue.add(temp.right);
                }
            }

            list.add(sum / size);
        }

        return list;
    }
}

/**
 * 时间复杂度 O(n)
 * 空间复杂度 O(n)
 */

https://leetcode-cn.com/problems/average-of-levels-in-binary-tree/