C++ Reverse Linked List accepted despite null pointer dereference

[senthil-lakshmikanth]

Bug description

I submitted the following C++ code on both NeetCode and LeetCode:

Code

class Solution {
public:
    ListNode* reverseList(ListNode* head) 
    {
        ListNode* prev = nullptr;
        ListNode* current = head;
        ListNode* next = current->next;    

        while(current != nullptr)
        {
            current->next = prev;
            prev = current;
            current = next;   
            next = current->next;         
        }

        return prev;
    }
};

• NeetCode: Accepted — 33/33 test cases

• LeetCode: Runtime Error — UndefinedBehaviorSanitizer

The issue

The issue is caused by accessing current -> next
Since current is nullptr, this is a null pointer dereference and causes undefined behavior.

ListNode* current = head;
ListNode* next = current->next;