#链表# #快慢指针#

function ListNode(val) {    this.val = val;    this.next = null;}
var deleteDuplicates = function(head) {   //边界条件判定，链表为空或者链表只有一个节点    if (!head || !head.next) return head;   //链表哑指针的设计    let dummyHead = new ListNode(null);    dummyHead.next = head;    //这里没有想到用两个指针解决，仅只想到遍历的逻辑    let prev = dummyHead, post = dummyHead.next, map = new Set();    while(post) {      if (!map.has(post.val)) {        map.add(post.val);        prev.next = post;        //这里逻辑就很顺了，找到没在散列表中的val，更新prev        prev = post; //prev = prev.next 也行      }      // post 自增      post = post.next;    }  	//截断 链表    prev.next = null;    return dummyHead.next;};