82. 删除排序链表中的重复元素 II（双指针）

给定一个排序链表，删除所有含有重复数字的节点，只保留原始链表中 没有重复出现 的数字。

示例 1:

输入: 1->2->3->3->4->4->5
输出: 1->2->5
示例 2:

输入: 1->1->1->2->3
输出: 2->3

示例代码

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if(head == null || head.next == null){
            return head;
        }
        ListNode hair = new ListNode(-head.val);
        hair.next = null;
        ListNode p = head;
        ListNode q = head;
        ListNode pre = hair;
        while(q != null){
            if(p.val == q.val){
                q = q.next;
            }else{
                if(p.next == q){
                    pre.next = p;
                    pre = p;
                    p = p.next;
                    q = q.next;
                }else{
                    p = q;
                    q = p.next;
                }
            }
        }
        if(p.next == q){
            pre.next = p;
            pre = p;
        }
        pre.next = null;
        return hair.next;
    }
}