member access within null pointer of type 'struct ListNode'

struct ListNode { int val; ListNode *next; ListNode(int x) : val(x), next(NULL) {} }; class Solution { public: bool hasCycle(ListNode *head) { if(head == NULL) return false; ListNode* walker = head; ListNode* runner = head; while(runner->next != NULL && walker->next != NULL){ walker = walker->next; runner = runner->next->next; if(walker == runner) return true; } return false; } }; 

I was practicing an interview code which seemed to be pretty simple. I have to return a bool that determines whether or not the singly-linked list has a cycle. I made two pointers walker which moves 1 step and runner which moves 2 steps every iteration.

But then this code gave me an error:

Line 15: member access within null pointer of type 'struct ListNode' 

What causes that error?

5 Answers

You only make sure that runner->next is not null, however after assignment

runner = runner->next->next;

runner can become null.

This should solve it:

bool hasCycle(ListNode *head) { if(head == NULL || head->next == NULL) { return false; } struct ListNode * walker = new ListNode(1); struct ListNode * runner = new ListNode(2); walker = head; runner = walker->next; while(walker != fast){ if(runner == NULL || runner->next == NULL) { return false; } walker = walker->next; runner = runner->next->next; } return true; } 

//- If a loop from circular path then there is hundred% chance that they will going to meet at some point so here we are taking a walker which move one step and a runner which move two step.

bool hasCycle(ListNode *head){ if(head == NULL || head->next == NULL) return false; struct ListNode *temp = head; struct ListNode *walker; struct ListNode *runner; walker = runner= head; while(temp ){ walker = walker->next; runner = runner->next->next; if(runner == walker) // as soon both get at same address we got return as true value. { return True; } temp = temp->next; } return false; 

}

The Below Code should work fine.
It is the classic Hare-Tortoise theorem, where the hare takes 2 steps(distance units) while the tortoise goes by 1.

I think you did not check the nullity of runner->next->next which has caused this error

bool hasCycle(ListNode *head) { if(head == NULL || head->next == NULL) { return false; } ListNode* tortoise=new ListNode(); ListNode* hare = new ListNode(); tortoise=head; hare=tortoise->next; while(tortoise != hare) { if(hare == NULL || hare->next == NULL) { return false; } tortoise=tortoise->next; hare=hare->next->next; } return true; } 

Sort answer is here with explanation

It returns the error because runner=runner->next->next can be NULL and you are checking runner->next!=NULL in while conditioning so you have to do a little change In your code In order to get your answer In while condition check for runner->next->next!=NULL and get the right answer.