637. Average of Levels in Binary Tree

637题求均值，注意List这个interface是没有offer方法的，只有Queue和Deque有。

public List<Double> averageOfLevels(TreeNode root) {
    List<Double> res = new ArrayList<>();
    Deque<TreeNode> queue = new ArrayDeque<>();

    if(root == null) return res;
    queue.offer(root);
    while (!queue.isEmpty()) {
        int size = queue.size();
        double sum = 0;
        for (int i = 0; i < size; i++) {
            TreeNode node = queue.poll();
            sum += node.val;
            if (node.left != null) queue.offer(node.left);
            if (node.right != null) queue.offer(node.right);
        }
        res.add(sum/size);
    }
    return res;
}