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;
    }