637. 二叉树的层平均值

class Solution {
    public List<Double> averageOfLevels(TreeNode root) {
        List<Double> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            double sum = 0;
            for (int i = 0; i < size; i++) {
                TreeNode cur = queue.removeFirst();
                sum += cur.val;
                if (cur.left != null) {
                    queue.add(cur.left);
                }
                if (cur.right != null) {
                    queue.add(cur.right);
                }
            }
            res.add(sum / size);
        }
        return res;
    }
}