Segmentation Fault after calling method exists() - c++

Question

I'm having trouble manipulating a doubly linked list. I'm pretty sure I have my addNode(), getCount(), printList(), and getNode() working fine, but when executing, I get to right around my exists() and deleteNode() methods and I get a segmentation fault. exists() checks to see if a specific node exists, and deleteNode() passes in an id, and if the id matches a node id, it deletes that node. I don't know which method is messing up. Here are both of them. This is also my first post so if it's confusing and not thorough I apologize in advance.
bool LinkedList::deleteNode(int id){
bool didDelete = false;
bool hasResult = false;
Node *position;
position = head;
while(position!=NULL && !hasResult){
if(id == position->data.id){
hasResult = true;
} else {
std::cout << "position->next" << std::endl;
position = position->next;
}
}
// Deletes head node
if (position->prev == NULL) {
head = head->next;
head->prev = NULL;
delete (position);
didDelete = true;
}
// Deletes middle node
else if (position->next != NULL) {
position->prev->next = position->next;
position->next->prev = position->prev;
position->next = NULL;
position->prev = NULL;
delete(position);
didDelete = true;
}
// Deletes the tail node
// else if(position->next == NULL){
else{
position->prev->next = NULL;
position->prev = NULL;
delete(position);
didDelete = true;
}
return didDelete;
} // End of deleteNode
bool LinkedList::exists(int id){
Node *position;
position = head;
bool gotNode = false;
while (position != NULL && !gotNode) {
if(id == position->data.id){
gotNode = true;
} else{
position = position->next;
}
}
return gotNode;
} // End of exists

Answer 1

Your exists() is fine, though it can be simplified a little:
bool LinkedList::exists(int id){
for(Node *position = head; position; position = position->next) {
if (id == position->data.id) {
return true;
}
}
return false;
}
Your deleteNode() is broken, though. If the specified id is not found by the while loop, position will end up NULL, and the subsequent access to position->prev will invoke undefined behavior.
It is also needlessly verbose, it can be greatly simplified:
bool LinkedList::deleteNode(int id){
for(Node *position = head; position; position = position->next){
if (id == position->data.id){
if (position->next) position->next->prev = position->prev;
if (position->prev) position->prev->next = position->next;
if (position == head) head = head->next;
/* uncomment this if you ever add such a field...
if (position == tail) tail = tail->prev;
*/
delete position;
return true;
}
}
return false;
}