Segmentation fault in skip list - c++

Question

I am implementing a skip list. The insertion function works fine and is able to print the list. But now I am trying to implement the "find" function and in this function I am getting a segmentation fault at the condition of while loop . When I debug the code , it works fine for all iterations except the last one. In the last iteration during condition check to enter in while loop it shows segmentation fault at this line precisely
while(x->rgt->key < searchKey)
I am not able to understand the reason because when I do handrun of the code everything seems fine. Please have a look and tell me what am I doing wrong ?
Also find function works fine if i search 1 and 2 but at 3 and 4 the fault occurs.
class SkipList
{
private:
struct node{
int key;
int data;
int level;
struct node* rgt = nullptr;
struct node* dwn = nullptr ;
node(int k, int value, int l):
key(k), data(value), level(l)
{}
};
//generates the ndde level in tha range [1,maxLevel).
int randomLevel() const;
//returns a set of pointers to the location at each node where new links are to be created
std::vector<node*> update(int searchKey) const ;
//creates a new node and returns a pointer to it
static node* makeNode(int key, int val, int level);
// Returns the first node for which node->key < searchKey is false
node* lower_bound(int searchKey) const;
const float probability;
const int maxLevel;
// head and tail vectors
vector<node*> head;
vector<node*> nil;
public:
SkipList();
// ~SkipList();
void insert(int searchKey, int val);
void find(int searchKey) const;
void erase(int searchKey);
void print() const;
};
SkipList::SkipList() :
probability(0.5), maxLevel(4)
{
head.resize(maxLevel, nullptr);
nil.resize(maxLevel,nullptr);
int headkey = std::numeric_limits<int>::min();
int nilkey = std::numeric_limits<int>::max();
for(int i = 0; i < maxLevel ;i++)
{
head[i] = new node(headkey,0,maxLevel-1);
nil[i] = new node(nilkey,0,maxLevel-1);
if(i>0)
{
head[i]->dwn = head[i-1];
nil[i] ->dwn = nil[i-1];
}
head[i]->rgt = nil[i];
}
}
void SkipList::find(int searchKey) const
{
node* x = head[maxLevel-1];
for(int i = maxLevel-1 ; i>= 0 ;i--)
{
while(x->rgt->key < searchKey)
{
x = x->rgt;
}
if(i != 0){x = x->dwn;}
}
if ((x->rgt->key == searchKey))
cout<<"Found"<<endl;
}
void SkipList::insert(int searchKey, int val)
{
vector <node*> preds = update(searchKey);
node* temp;
const int newLevel = randomLevel();
for(int i = 0; i< newLevel; i++)
{
node* ptr = makeNode(searchKey,val, newLevel-1);
temp = preds[i]->rgt;
preds[i]->rgt = ptr;
ptr->rgt = temp;
}
}
void SkipList::print() const
{
node *ptr = head[0]->rgt;
while(ptr->rgt != nullptr)
{
cout<<"Key: "<<ptr->key<<" Data: "<<ptr->data<<" Level: "<<ptr->level<<endl;
ptr = ptr->rgt;
}
}
int SkipList::randomLevel() const
{
int v = 1;
while (((double)std::rand() / RAND_MAX) < probability &&
v < maxLevel)
{
v++;
}
return v;
}
SkipList::node* SkipList::makeNode(int key, int value, int level)
{
return new node(key, value, level);
}
std::vector<SkipList::node*>SkipList::update(int searchKey) const
{
int level = head[0]->level;
std::vector<node*> result(level,nullptr);
node* x ;
for(unsigned int i = level;i-- >0;)
{
x = head[i];
while(x->rgt->key < searchKey)
{
x = x->rgt;
}
result[i]= x;
}
return result;
}
int main()
{
SkipList s;
int x,y;
for(int i = 1;i< 5;i++)
{
s.insert(i,i);
}
s.print();
cout<<endl;
s.find(3);
return 0;
}

Answer 1

A simple debug shows this in this loop:
while(x->rgt->key < searchKey)
{
x = x->rgt;
}
The value of x becomes NULL.
I am guessing that this is a case where you try to add an element at the end of the list.
If so, then you should come up with a different solution for this case.
For example:
while(x->rgt && x->rgt->key < searchKey)
{
x = x->rgt;
}
if (x->rgt)
{
// Use your original code
}
else
{
// Handle this special case
}