题目：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.

题目解答：使用三个指针，p标明已经处理好的链表部分；q用来指代当前待比较的节点；r用来依次遍历是否有重复的元素。

同样使用虚拟头结点，来避免过多的判断。

代码如下：

/**

 * 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) {

        ListNode * Head = new ListNode(-1);

        Head -> next = head;

        ListNode *p = Head;

        ListNode *q = p -> next;

        while(q != NULL)

        {

            ListNode *r = q -> next;

            while( (r != NULL) && (r -> val == q -> val) )

            {

                ListNode *tmp = r -> next;

                delete r;

                r = tmp;

            }

            q -> next = r;

            p -> next = q;

            p = q;

            q = q -> next;

        }

        ListNode *tmp = Head -> next;

        delete Head;

        return tmp;

    }

};