Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {

    public List<Double> averageOfLevels(TreeNode root) {
        if (root == null) return null;

        List<Double> res = new ArrayList<>();
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(root);

        while (!queue.isEmpty()) {
            double sum = 0, size = queue.size();
            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;
    }
}