leetcode-Remove Duplicates from Sorted List II

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.

For example,
Given 1->2->3->3->4->4->5, return 1->2->5.
Given 1->1->1->2->3, return 2->3.

思路：双指针，与-Remove Duplicates from Sorted List类似，只是需要注意细节

代码：

ListNode *deleteDuplicates(ListNode *head) {
        if(head == NULL) return head;
ListNode *pre=NULL;
ListNode *p=head;
ListNode *q=p->next;
if(q == NULL) return head;


while(p!=NULL && q!=NULL)
{
if(p->val == q->val)
{
q=q->next;
}
else
{
if(pre == NULL)
{
if(p->next != q)
{
head=q;
p=head;
q=q->next;
pre=NULL;
}
else
{
pre=p;
p=q;
q=q->next;
}
}
else
{
if(p->next != q)
{
pre->next=q;
p=q;
q=q->next;
}
else
{
pre=p;
p=q;
q=q->next;
}
}
}
}
if(p->next == q)
{
return head;
}
else
{
if(pre == NULL)
{
return pre;
}
else
{
pre->next=NULL;
}
}
return head;
    }