从二叉搜索树中删除？

Question

I'm trying to delete from a binary search tree and keep getting this error in debugger and am not sure what to do to correct it. Is this code correct?

Program received signal EXC_BAD_ACCESS, Could not access memory. Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000000 0x00007fff8cc17fe2 in std::string::compare ()

void remove( const Comparable & x, BinaryNode *& t )
{
    if (t != NULL)
    {
        if(t->element.find(x) != std::string::npos)
        {
            if( t->left != NULL && t->right != NULL ) // Two children
            {
                t->element = findMin( t->right )->element;
                remove( t->element, t->right);
            }
            else
            {
                BinaryNode *oldNode = t;
                t = ( t->left != NULL ) ? t->left : t->right;
                delete oldNode;
                cout << "Successly deleted!" << endl;
            }
        }
        if(x < t->element)
        {
            remove(x, t->left);
        }
        else
        {
            remove(x, t->right);
        }
    }
    else 
    {
        cout << x << "<-could not delete?" << endl;            
    }     
}

Answer 1

First, compile this with debug settings, then run it under a debugger . I can all-but-guarantee it will trip exactly where your failure case is.

On that note, I speculate it is this line :

if(x < t->element)  // <==== here
{
    remove(x, t->left);
}
else
{
    remove(x, t->right);
}

For some reason your logic before this takes the following deductions:

• Neither left NOR right are null
• Only left OR right are null

You don't account for the case when both left and right are null, such as would be the case in a tree leaf node. Consequently this, taken from your else condition:

BinaryNode *oldNode = t;
t = ( t->left != NULL ) ? t->left : t->right;
delete oldNode;
cout << "Successly deleted!" << endl;

In the case of a leaf node, will leave t set to null, which is immediately dereferenced by the code at the beginning of this answer.

You need to rework your logic for this, and if code prior to a dereference can invalidate the pointer being dereferenced, you need to check it first .

Finally if you're wondering what the hint was that is the offending line, the specific error your getting reports string comparison is dereferencing a null ptr. String comparison isn't done anywhere else in this function except through that operator < overload.