double max = Integer.MIN_VALUE;
TreeNode maxNode = null;

public TreeNode maximumAverageSubtree(TreeNode root) {
    if (root == null) return null;
    helper(root);
    return maxNode;
}

private double[] helper(TreeNode root) {
    if (root == null) return new double[] {0, 0};

    double curTotal = root.val;
    double count = 1;
    for (TreeNode child : root.children) {
        double[] cur = helper(child);
        curTotal += cur[0];
        count += cur[1];
    }
    double avg = curTotal / count;
    if (count > 1 && avg > max) { //taking "at least 1 child" into account
        max = avg;
        maxNode = root;
    }
    return new double[] {curTotal, count};
}