Leetcode题目：Remove Duplicates from Sorted List

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;
    }
};