LeetCode 83. Remove Duplicates from Sorted List

1->1->2

1->2

1->1->2->3->3

1->2->3

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     struct ListNode *next;
* };
*/
struct ListNode* deleteDuplicates(struct ListNode* head) {
struct ListNode *pre,*p;
if(head==NULL){
return head;
}
while(head!=NULL && head->next!=NULL && head->next->val==head->val){
head=head->next;
}
pre=head;
p=head->next;
while(p){
if(p->val==pre->val){
pre->next=p->next;
p=p->next;
}else{
pre=p;
p=p->next;
}
}
return head;
}