Remove Duplicates from Sorted List II [LeetCode]

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.

Solution:

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

         //remove the head
         ListNode * same_node = NULL;
         while(head != NULL) {
             if(same_node == NULL) {
                 if(head->next != NULL && head->val == head->next->val){
                     same_node = head;
                     head = head->next;
                 }else {
                     break;
                 }
             }else {
                 if(head->val == same_node->val)
                     head = head->next;
                 else
                     same_node = NULL;
             }
         }

         if(head == NULL || head->next == NULL)
             return head;  

         ListNode * pre = head;
         ListNode * same = NULL;
         ListNode * current = pre->next;
         while(current != NULL) {
             if(same == NULL) {
                if(current->next != NULL && current->val == current->next->val){
                     same = current;
                     pre->next = current->next;
                 }else{
                     pre = pre->next;
                 }
                 current = current->next;
             }else {
                 if(current->val == same->val){
                     pre->next = current->next;
                     current = current->next;
                 }else {
                     same = NULL;
                 }
             }
         }

         return head;
     }