Remove Duplicates from Sorted List II

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* deleteDuplicates(ListNode* head) {
if(head ==NULL) return head;
if(head->next ==NULL) return head;
ListNode *res = head;

int head_val;

while(head){
head_val = head->val;
if(head->next && head ->next->val == head_val){
while(head && head->val == head_val)  //去掉开头重复的
{
head =head->next;
}
}
else break;
}
res=head;

//如果重复的部分不是在开头，需要记录pre结点。这里特别要注意逻辑性，head只有在后续结点不是重复结点的情况下，才可以往后移动
while(head){
if(head->next == NULL) break;
ListNode* p_i = head->next;
int next_val = p_i->val;
if(p_i!=NULL &&p_i->next!=NULL && p_i->next->val == next_val){
while(p_i && p_i->val ==next_val)
{
p_i =p_i->next;
}
if(p_i == NULL) {head->next=NULL;break;}
head->next = p_i;
}
else head = p_i;
}

return res;
}

};