leetcode 082 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

.

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/**
* 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) {
ListNode *l = head, *pre=head, *ret=NULL, *cur=NULL;

if(head == NULL) return head;

int cnt = 1;

for(ListNode *next=l->next; next!=NULL; next=next->next) {
if(pre->val == next->val) {
cnt++;
pre = next;
continue;
}

if(cnt == 1)
{
if(ret == NULL) {
ret = pre;
}else {
cur->next = pre;
}
cur = pre;
cur->next=NULL;
//cout << cur->val << endl;
}

cnt = 1;
pre = next;
}

if(cnt == 1) {
if(ret==NULL) {
ret=pre;
} else {
cur->next=pre;
}
cur=pre;
cur->next=NULL;
}
return ret;
}
};