BST删除方法中的错误？

Question

我认为此代码中存在一个错误。我构建了两个不同的BST，并在两者上执行了删除方法。一个工作，另一个没工作。

public void delete(int data) {
    root = delete(root, data);
}

private Node delete(Node node, int data) {
    if(node == null)    throw new IllegalStateException("Empty BST. Cannot delete.");

    if(data < node.data) {
        delete(node.left, data);
    }
    else if(data > node.data) {
        delete(node.right, data);
    }
    else {
        if(node.left == null)
            return node.right;
        else if(node.right == null)
            return node.left;
        else {
            // getting data from the rightmost node in the left subtree
            // because its greater than all the elements the left subtree of the deleted node
            // and smaller than all the elements in the right subtree of the deleted node
            node.data = retrieveData(node.left);
            // delete the rightmost node in the left subtree
            node.left = delete(node.left, node.data);
        }
    }
    return node;
}

private int retrieveData(Node node) {
    while(node.right != null)
        node = node.right;
    return node.data;
}

主要

中的代码

    BinarySearchTree bst = new BinarySearchTree(5, 2, 12, -4, 3, 9, 21, 19, 25);
    System.out.println(bst.levelOrder());
    bst.delete(12);
    System.out.println(bst.levelOrder());

    bst = new BinarySearchTree(4, 2, 6, -1, 8, 7);
    System.out.println(bst.levelOrder());
    bst.delete(6);
    System.out.println(bst.levelOrder());

输出

[5, 2, 12, -4, 3, 9, 21, 19, 25]
[5, 2, 9, -4, 3, 21, 19, 25]
[4, 2, 6, -1, 8, 7]
[4, 2, 6, -1, 8, 7]

节点（12）已被删除，BST维持其结构。 但是，节点（6）尚未删除。