runtime error: member access within null pointer of type ‘struct ListNode‘

Wrong topic: delete duplicate elements in the sorted list

例如 1->1->2
输出 1->2

The cause of the error: Trying to use a null pointer.
Solution: Increase the judgment condition, and the order of judgment cannot be changed. Exclude references to null pointers.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
    
    
public:
    ListNode* deleteDuplicates(ListNode* head) {
    
    
       /* if (head == nullptr || head->next == nullptr) {
            return head;
        }*/
         if (head->next == nullptr || head == nullptr) {
    
    
            return head;
        }
        ListNode* cur = head->next;
        ListNode* node = head;
        while (cur != nullptr) {
    
    
            if (cur->val == node->val) {
    
    
                node->next = cur->next;
                cur = cur->next;
            }
            else {
    
    
                cur = cur->next;
                node = node->next;
            }
        }
        return head;
    }
};

The above code seems to be logically correct, but when judging a null pointer, an error is reported.
The error is this paragraph:

runtime error: member access within null pointer of type 'struct ListNode'

The meaning of this sentence is to try to use a null pointer. An error occurred when judging the following statement.

if (head->next == nullptr || head == nullptr) 

We can see that if it headis a null pointer, it head->nextis illegal in itself, so an error will be reported, but if we change the order, it will be correct
if (head == nullptr || head->next == nullptr).

This is a mistake when doing the question, I hope it can be remembered.