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

.

/**
* 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) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
ListNode *cur = head, *pre = NULL, *next = NULL;
if (head) {
next = head->next;
} else {
return NULL;
}
bool has_same = false;

//寻找head元素
while (next) {
if (cur->val == next->val) {
has_same = true;
next = next->next;
} else {
if (has_same) {
cur = next;
next = next->next;
has_same = false;
} else {

has_same = false;
break;
}
}
}
if (has_same) {
return NULL;
}

head = cur;
cur = next;
pre = head;

if (next) {
next = next->next;
while (next) {
if (cur->val == next->val) {
has_same = true;
next = next->next;
} else {
if (has_same) {
cur = next;
next = next->next;
has_same = false;
} else {
pre->next = cur;
pre = cur;
cur = next;
next = next->next;
has_same = false;
}
}
}

if (has_same) {
pre->next = NULL;
}  else {
pre->next = cur;
}
}
return head;
}
};