public class TreeNode {
    public int val;
    public TreeNode? left;
    public TreeNode? right;
    public TreeNode(int val = 0, TreeNode? left = null, TreeNode? right = null) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}


public class Solution {
    private class Level {
        long sum;
        int counter;

        public Level() {
            sum = 0;
            counter = 0;
        }

        public void Add(int x) {
            sum += x;
            counter++;
        }

        public double Average {
            get => sum / (double)counter;
        }
    }

    private List<Level> AverageOfLevels(List<Level> averages, TreeNode? node, int level) {
        if (node == null) {
            return averages;
        }

        if (level == averages.Count) {
            averages.Add(new Level());
        }
        averages[level].Add(node.val);

        AverageOfLevels(averages, node.left, level + 1);
        AverageOfLevels(averages, node.right, level + 1);

        return averages;
    }

    public IList<double> AverageOfLevels(TreeNode? root)
        => AverageOfLevels(new List<Level>(), root, 0).Select(level => level.Average).ToList();

    public static void Main() {
        var s = new Solution();

        foreach (var a in s.AverageOfLevels(new TreeNode(3, new TreeNode(9), new TreeNode(20, new TreeNode(15), new TreeNode(7))))) {
            Console.Write($"{a} ");
        }
        Console.WriteLine();
    }
}