member access within null pointer of type, C programming is Palindrome

Question

I am trying to solve isPalindrome() question on LeetCode using non recurisve solution, When i run this code using VSCode it runs and gives me the right output, but when i run it in LeetCode compiler it gives me the error mentioned below.

Can you help me solve this problem ? and is there's any modifications on my code may help me solve the problem in a better way ?

bool isPalindrome(struct ListNode* head){ struct ListNode * NewHead, *MidList, *EndList; NewHead = EndList = MidList = head; struct ListNode *ptr_SecondHalf, *ptr_FirstHalf; while (EndList->next != NULL) { EndList = EndList->next->next; MidList = MidList->next; if (EndList->next == NULL || EndList->next->next == NULL){ //ODD if (EndList->next == NULL){ ptr_SecondHalf = MidList->next; break; } //EVEN if (EndList->next->next == NULL){ ptr_SecondHalf = MidList->next; break; } } } MidList->next = NULL; ptr_FirstHalf = head; // Reverse SecondHalf struct ListNode* ptr_SecondHalf_Reversed = NULL; struct ListNode* current = ptr_SecondHalf; struct ListNode* next = NULL; while (current != NULL) { // Store next next = current->next; // Reverse current node's pointer current->next = ptr_SecondHalf_Reversed; // Move pointers one position ahead. ptr_SecondHalf_Reversed = current; current = next; } while (ptr_FirstHalf->next != NULL && ptr_SecondHalf_Reversed != NULL){ if (ptr_FirstHalf->val == ptr_SecondHalf_Reversed->val){ if (ptr_FirstHalf->next->next != NULL || ptr_SecondHalf_Reversed ->next != NULL){ ptr_FirstHalf = ptr_FirstHalf->next; ptr_SecondHalf_Reversed = ptr_SecondHalf_Reversed ->next ; } else { break; } }else{ printf("false \n"); return false; } } printf("true \n"); return true; }

Line 12: Char 21: runtime error: member access within null pointer of type 'struct ListNode' [solution.c]

You check EnlistNext and EndlistNextNext. But you do not check Endlist. — Yunnosch
– Yunnosch, Commented Jan 4, 2021 at 15:42
Please update your minimal reproducible example to that (possible while there is no answer yet). — Yunnosch
– Yunnosch, Commented Jan 4, 2021 at 15:59
Why do you sometimes use a space before the -> operator? Please don't do that. It's confusing to read and doesn't match any other C code style I've seen in my entire career. — Jonathon Reinhart
– Jonathon Reinhart, Commented Jan 4, 2021 at 16:01

Mahmoud Elnahal · Accepted Answer · 2021-01-04 16:58:40Z

your solution idea isn't wrong, but is too hard to implement and don't have any bug, I suggest my solution or suggest to write one method which will return reverse list and then just compare these two.

bool isPalindromeRec(struct ListNode* left, struct ListNode* right) { if(right == NULL) return true; if(isPalindromeRec(left, right->next) && left->val == right->val) { left = left->next; return true; } return false; } bool isPalindrome(struct ListNode* head){ if(head == NULL) return true; struct ListNode * left = head; struct ListNode * right = head; bool ans = isPalindromeRec(left, right); // happy now Vlad from Moscow? if(ans) printf("true\n"); else printf("false\n"); return ans; }

this solution I just came up, should be right - it just compares each node from left and from right, but I think this is not time which will change a lot. EDIT:

bool isPalindrome(struct ListNode *head) { if(head == NULL|| head->next== NULL) return true; //find list center struct ListNode *fast = head; struct ListNode *slow = head; while(fast->next != NULL && fast->next->next != NULL){ fast = fast->next->next; slow = slow->next; } struct ListNode *secondHead = slow->next; slow->next = NULL; //reverse second part of the list struct ListNode *p1 = secondHead; struct ListNode *p2 = p1->next; while(p1 != NULL && p2 != NULL){ struct ListNode *temp = p2->next; p2->next = p1; p1 = p2; p2 = temp; } secondHead->next = NULL; //compare two sublists now struct ListNode *p = (p2 == NULL?p1:p2); struct ListNode *q = head; while(p!=NULL){ if(p->val != q->val) return false; p = p->next; q = q->next; } return true; }

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