LeetCode: Remove Duplicates from Sorted List

Problem:

Given a sorted linked list, delete all duplicates such that each element appear only once.

For example,
Given 1->1->2, return 1->2.
Given 1->1->2->3->3, return 1->2->3.

/**
 * 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 == NULL)
            return head;
        
        ListNode* cur = head;
        ListNode* pre = head->next;
        ListNode* del = NULL;
        while(pre != NULL)
        {
            if (cur->val == pre->val)
            {
                del = pre;
                pre = pre->next;
                cur->next = pre;
                delete del;
            }
            else
            {
                cur = cur->next;
                pre = pre->next;
            }
        }
        return head;
    }
};