LeetCode 637 Average of Levels in Binary Tree

给予树：
    3
   / \
  9  20
    /  \
   15   7
返回: [3, 14.5, 11]

/**
 * 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) {
        List<Double> result = new ArrayList<>();
        
        if (root == null) {
            return result;
        }

        Queue<TreeNode> queue = new ArrayDeque<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            double sum = 0;
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.remove();
                sum += node.val;

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

                if (node.right != null) {
                    queue.add(node.right);
                }
            }
            result.add(sum / size);
        }
        return result;
    }
}