Trying to delete a node from a binary search tree

Question

I'm getting a memory error when I try to display my tree after a node was deleted.
This is my remove (delete) method:

void binarySearchTree::remove(int x, node * r)
{
    bool found = false;
    node * previous = NULL;

    if (root == NULL)
    {cout << "Tree is empty. Nothing to remove." << endl; return;}

    while (!found)
    {
        if (x < r->data)
        {
            previous = r;
            r = r->left;
        }
        else if (x > r->data)
        {
            previous = r;
            r = r->right;
        }
        else
            found = true;
    }


    if (r->left == NULL && r->right == NULL)        //case 1: node to be deleted is a leaf (no children)
    {
        delete r;
        return;
    }
    else if(r->left == NULL && r->right != NULL)    //case 2: node only has a right child
        previous->right = r->right;
    else if (r->left != NULL && r->right == NULL)   //case 2: node only has a left child
        previous->left = r->left;
    else
    {                                               //case 3: node has two children
        node * minNode = findMinNode(r->right);     //finds min node in the right sub tree
        r->data = minNode->data;
        delete minNode;
        return;
    }
    delete r;
}

My findMinNode method:

binarySearchTree::node * & binarySearchTree::findMinNode(node * r)
{
    if (r == NULL)      //if tree is empty
        return r;

    if (r->left == NULL && r->right == NULL)
        return r;
    else if (r->left != NULL)
        return findMinNode(r->left);
    else
        return findMinNode(r->right);
}

My display method (using preorder traversal):

void binarySearchTree::display(node * r)
{
    if (r == NULL)
        return;

    display(r->left);
    cout << r->data << endl;
    display(r->right);
}

I'm using a public display() method which then calls this private display(node * r) method.

I know the problem is when I use delete because when I stepped through the code and got to my display() method, when it checked to see if r== NULL on the node I just deleted, the address was not NULL with an address of 0x000000000 but instead had 0x000000001 . Because of this my program would crash. I've never encountered this problem before. Any help would be greatly appreciated.

I should add that I inserted these values in this exact order: 10, 5, 34, 2, 8, 12, 25, 6, 18, 27, 38, 11. I was trying to remove the node with value 12 because it had two children.

Answer 1

When you delete a node you need to NULL out the pointer that points to it, which in your example can be root, previous->left, or previous->right. If you change previous to be a node ** and have it point at the previous pointer (initialized to &root) then you can just say *previous = null or *previous = r->right .