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;
};