LeetCode - 637. Average of Levels in Binary Tree

/**
 * 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<>();
        Queue<TreeNode> q = new LinkedList<TreeNode>();
        if (root == null) return result;
        q.add(root);
       
        while (!q.isEmpty()) {
            int size = q.size();
            double sum = 0.0;
            for (int i = 0; i < size; i++) {
                TreeNode node = q.poll();
                sum += node.val;
                if (node.left != null) q.offer(node.left);
                if (node.right != null) q.offer(node.right);
            }
            result.add(sum / size);
        }
        
        return result;
    }
}