【链表】Remove Duplicates from Sorted List II（三指针）

题目：

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.

For example,
Given 1->2->3->3->4->4->5, return 1->2->5.
Given 1->1->1->2->3, return 2->3.

思路：

找到重复的项，记录下他们前面的节点，然后删除。

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var deleteDuplicates = function(head) {
    if(head==null||head.next==null){
        return head;
    }
    var l=head,r=head.next,rpre=null;
    var tempHead=new ListNode(0);
    tempHead.next=head;
    var pre=tempHead;
    while(r!=null){
        if(l.val==r.val){
            while(r!=null&&r.val==l.val){
                rpre=r;
                r=r.next;
            }
            pre.next=r;
            if(r==null){
                return tempHead.next;
            }else{
                l=r;
                r=r.next;
            }
        }else{
            l=l.next;
            r=r.next;
            pre=pre.next;
        }
    }
    
    return tempHead.next;
};