LeetCode 637 average of levels in binary tree

just use level traverse and calculate the average of each level.

class Solution {
    public List<Double> averageOfLevels(TreeNode root) {
        List<Double> final_res = new ArrayList<>();
        //List<List<Integer>> res = new ArrayList<>();
        if (root == null) return final_res;
        Queue<TreeNode> queue = new LinkedList<>();
        
        TreeNode cur = root;
        queue.offer(cur);
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                cur = queue.poll();
                list.add(cur.val);
                if (cur.left != null) queue.offer(cur.left);
                if (cur.right != null) queue.offer(cur.right);
            }
            Double num = average(list);
            final_res.add(num);
            
        }
        return final_res;
    }
    
    private Double average(List<Integer> list) {
        
        int size = list.size();
        
        Double sum = 0.0;
        for (Integer num: list) {
            sum += num;
        }
        return sum / size;
    }
}