member access within null pointer of type 'struct ListNode'

Question

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?

Answer 1

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

runner = runner->next->next;

runner can become null.

Answer 2

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;
}

Answer 3

//- 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;

}

Answer 4

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;
}

Answer 5

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.

Happy Coding :>)