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

.

class Solution {
public:
ListNode *deleteDuplicates(ListNode *head) {
if (head == NULL || head->next == NULL)
return head;

ListNode *p1 = head->next;
ListNode *p2 = head;
ListNode *p3 = new ListNode(0);
p3->next = head;

int flag = 0;
while (p1 != NULL) {
if (p1->val != p2->val && flag == 0) {
p3->next->val = p2->val;
p3 = p3->next;
}
else if (p1->val != p2->val && flag == 1) {
flag = 0;
}
else {
flag = 1;
}

p2 = p1;
p1 = p1->next;
}

if (flag == 0) {
p3->next->val = p2->val;
p3 = p3->next;
} else if (p3->next == head) {
return NULL;
}

p3->next = NULL;

return head;
}
};