Pointer is modified unexpectedly - c++

Question

I'm coding the Red-Black-Tree by following Introduction to Algorithms. And when it comes to the right-rotate function, one of my pointer was modified unexpectedly.
void RBTree::RightRotate(pNode &z)
{
pNode l=z->lChild;
z->lChild = l->rChild; //link z->l-r to z as a lChild
if(l->rChild != nil)
l->rChild->p = z;
l->p = z->p; //when pass the tree's root as parameter
//z,after executing this line, z was
//modified to pointing to nil
//(known from debug window), why?
if(z->p == nil)
root = l;
else if (z == z->p->lChild)
z->p->lChild = l;
else
z->p->rChild = l;
l->rChild=z;
z->p=l;
}
And just as Introduction to Algorithms goes, I set nil pointer in my class.
Since only when root was passed as parameter z can cause this problem, I defer it's something wrong with nil and root.The following is the whole code.
:
#include
#include
#include
#define RED 1
#define BLACK 0
using namespace std;
typedef struct Node
{
int color, val;
struct Node* lChild, *rChild, *p;
Node(int val_, struct Node* tmp)
{color=RED, val=val_,lChild=rChild=p=tmp;}
Node(){color=BLACK, val=0,lChild=rChild=p=NULL;}
}Node,*pNode;
class RBTree
{
public:
RBTree()
{
root=NULL;
}
~RBTree()
{
Free_(root);
}
void Free_(pNode p)
{
if(p!=NULL)
{
Free_(p->lChild);
Free_(p->rChild);
delete(p);
p=NULL;
}
}
pNode FindMin() const
{
pNode p=root;
if (p)
while(p->lChild)
p=p->lChild;
return p;
}
pNode FindMax() const
{
pNode p=root;
if (p)
while(p->rChild)
p=p->rChild;
return p;
}
void Print(pNode p, int n) const
{
if(!p)
return;
Print(p->rChild,n+2);
for(int i=0;ivalcolor==1)?"R":"B")lChild,n+2);
}
pNode Root() {return root;}
void Insert(int const& val_);
void InsertFixup(pNode &z);
void LeftRotate(pNode &p);
void RightRotate(pNode &p);
pNode Search(pNode const &p, int x) const;
void Delete(pNode &p, int x);
private:
pNode root;
static pNode nil;
};
pNode RBTree::nil = new Node();
/*
Inserting pipiline:
Insert node into RBTree, then fixup.
In the fixup procedure , rotate may be called
*/
void RBTree::InsertFixup(pNode &z)
{
while (z->p->color == RED)
{
pNode y;
if (z->p == z->p->p->lChild)
{
y=z->p->p->rChild;
if(y->color == RED)
{
z->p->color = BLACK;
y->color = BLACK;
z->p->p->color = RED;
z = z->p->p;
}
else
{
if(z==z->p->rChild)
{
z = z->p;
LeftRotate(z);
}
z->p->color = BLACK;
z->p->p->color = RED;
RightRotate(z->p->p);
}
}
else
{
y=z->p->p->lChild;
if(y->color == RED)
{
z->p->color = BLACK;
y->color = BLACK;
z->p->p->color = RED;
z=z->p->p;
}
else
{
if(z==z->p->lChild)
{
z = z->p;
RightRotate(z);
}
z->p->color = BLACK;
z->p->p->color = RED;
LeftRotate(z->p->p);
}
}
}
root->color = BLACK;
}
void RBTree::Insert(int const &val_)
{
pNode z=new Node(val_, nil); //set val, lChild, rChild, p to NULL, color to RED
pNode x = root;
pNode pre = nil;
if(NULL == root)
root = z;
else
{
while (x!=nil)
{
pre=x;
x=(x->valrChild:x->lChild;
}
z->p = pre;
if (z->val val)
pre->lChild = z;
else
pre->rChild = z;
}
InsertFixup(z);
}
//left-rotate
//denote z, z's rChild == r
//parent changed: z, r, r's lChild
//child chagend: z->rChild, r->lChild, z->p->lChild/rChild
void RBTree::LeftRotate(pNode &z)
{
pNode r=z->rChild;
z->rChild = r->lChild; //link p->r-l to p as a r
if(r->lChild != nil)
r->lChild->p = z;
r->p = z->p;
if(z->p == nil)
root = r;
else if (z == z->p->lChild)
z->p->lChild = r;
else
z->p->rChild = r;
r->lChild=z;
z->p=r;
}
// right-rotate
void RBTree::RightRotate(pNode &z)
{
pNode l=z->lChild;
z->lChild = l->rChild; //link p->l-r to p as a l
if(l->rChild != nil)
l->rChild->p = z;
l->p = z->p;
if(z->p == nil) //link parent from top to bottom
root = l;
else if (z == z->p->lChild)
z->p->lChild = l;
else
z->p->rChild = l;
l->rChild=z;
z->p=l;
}
int main()
{
int a[9] = {11, 2, 14, 1, 7, 15, 5, 8, 4};
RBTree tr;
for(int i=0;i
Am I using nil in a wrong way?
Thanks.

Answer 1

Nill is not a C++ expression. However some environments might include it, just using it as an alias for NULL or 0. If you use C++11, I would recommend nullptr
Try to have a look whether
l->p < z && z< l->p + sizeof(z)
If that is the case, you are assigning to z.