题解 | #删除有序链表中重复的元素-II#

主要的思路：创建一个虚拟头结点，目的就是为了找到前驱节点。在移动的过程中如果满足，当前节点的值不等于前驱节点的值，并且不等于后驱节点的值（后驱节点可能为空，为空的情况是满足条件的）。就报该点保留下来。

/**
 * struct ListNode {
 *  int val;
 *  struct ListNode *next;
 * };
 */

class Solution {
  public:
    /**
     *
     * @param head ListNode类
     * @return ListNode类
     */
    ListNode* deleteDuplicates(ListNode* head) {
        ListNode* dummyHead = (ListNode*)malloc(sizeof(ListNode));
        dummyHead->val = 1001;
        dummyHead->next = head;
        ListNode* pre = dummyHead, *cur = head, *dummy = dummyHead;
        while (cur != NULL) {
            if (pre->val != cur->val) {
                if (cur->next == NULL || cur->next->val != cur->val) {
                    dummy->next = cur;
                    dummy = cur;
                }
            }
            pre = cur;
            cur = cur->next;
        }
        dummy->next = NULL;
        return dummyHead->next;
    }
};