[LeetCode] 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) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if (!head) return NULL;

ListNode *prev = NULL;
ListNode *curr = head;
bool flag = false;

while (curr) {
if (curr->next && curr->val == curr->next->val) {
curr->next = curr->next->next;
flag = true;
} else if (flag) {
if (!prev) head = curr->next;
else prev->next = curr->next;

curr = curr->next;
flag = false;
} else {
prev = curr;
curr = curr->next;
}
}

return head;
}
};