leetcode82——Remove Duplicates from Sorted List II

题目大意：删除升序链表中的结点，使得链表最后只剩下从未重复过的数字

分析：链表删除操作。和leetcode83不同的是这道题用到了伪结点。

代码：

/**
* 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 || head->next == NULL)
            return head;
        ListNode *dummy = new ListNode(0);
        dummy->next = head;
        ListNode *pre = dummy;
        ListNode *p = head;
        while (p) {
            if (p->next && p->val == p->next->val) {  //如果p和p的下一个结点重复，进行删除操作
                int mark = p->val;
                while (p && p->val == mark) {
                    pre->next = p->next;
                    delete p;
                    p = pre->next;
                }
            }
            else {
                pre = p;
                p = p->next;
            }
        }
        return dummy->next;
    }
};