Write a method that checks if a binary tree is complete

public Boolean isCompleteTree(TreeNode node) {
 int numberNodes = countNodes(node);
    return isComplete(node, 0, numberNodes);
}

private Boolean isComplete(TreeNode node, int index, int numberNodes) {
    if (node == null)
        return true;

    if (index > numberNodes)
        return false;

    return isComplete(node.left(), 2 * index + 1, numberNodes) &&
            isComplete(node.right(), 2 * index + 2, numberNodes);
}

private Integer countNodes(TreeNode node) {
    if (node == null) {
        return 0;
    }
    return 1 + countNodes(node.left()) + countNodes(node.right());
}