LeetCode 637: Average of Levels in Binary Tree

1. /**
    * 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 LinkedList<>();
           queue.offer(root);
           double sum = 0.0;
           int num = 0;
           
           while (!queue.isEmpty()) {
               sum = 0.0;
               num = queue.size();
               int i = num;
               while (i-- > 0) {
                   TreeNode current = queue.poll();
                   if (current.left != null) {
                       queue.offer(current.left);
                   }
               
                   if (current.right != null) {
                       queue.offer(current.right);
                   }
               
                   sum += current.val;
               }
               result.add(sum / num);
           }
           return result;
       }
   }