Java中二分查找树的add方法

public class BinarySearchTree {
    private Node root;

    private class Node {
        private int key;
        private Node left;
        private Node right;

        public Node(int key) {
            this.key = key;
            this.left = null;
            this.right = null;
        }
    }

    public void add(int key) {
        root = addNode(root, key);
    }

    private Node addNode(Node node, int key) {
        if (node == null) {
            return new Node(key);
        }

        if (key < node.key) {
            node.left = addNode(node.left, key);
        } else if (key > node.key) {
            node.right = addNode(node.right, key);
        }

        return node;
    }
}