struct ListNode* deleteDuplicates(struct ListNode* head)
{
     struct ListNode *p,*q,*t;
    p=head;
    q=head->next;
    while(p!=NULL)
    {
        if(p->val==q->val)
        {
            p->next=q->next;
            t=q;
            q=q->next;
            free(t);
        }
        else
        {
            p=q;
            q=q->next;
        }
    }
    return head;
}

Error Message Shown as: Line 19: Char 21: runtime error: member access within null pointer of type 'struct ListNode' [solution.c]

CodePudding user response：

The logic is "backwards", it tries to check if the local element is the same as the next element, instead of checking if it's the same as the previous.

Consider a list of one element, and it's obvious how it breaks when dereferencing head->next->val.

CodePudding user response：

If the pointer head or q is a null pointer then these statements

q=head->next;

if(p->val==q->val)

and

q=q->next;

invoke undefined behavior.

The function can be defined the following way

struct ListNode * deleteDuplicates( struct ListNode *head )
{
    for ( struct ListNode *p = head; p != NULL && p->next != NULL; )
    {
        if ( p->val == p->next->val )
        {
            struct ListNode *tmp = p->next;
            p->next = p->next->next;
            free( tmp );
        }
        else
        {
            p = p->next;
        }
    }

    return head;
}