leetcode Remove Duplicates from Sorted Lists

/**
 * 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) {
        if(head == NULL){
            return head;
        }

        ListNode* tmp = head;
        ListNode* next = tmp->next;
        //ListNode* prev = tmp;
        while(next){
            if(next->val == tmp->val){
                ListNode*tmpnext = next->next;
                tmp->next = tmpnext;
                delete next;
                next = tmpnext;
            }
            else{
                tmp = next;
                next = next->next;
            }
        }
        return head;
    }
};