/*
 * function ListNode(x){
 *   this.val = x;
 *   this.next = null;
 * }
 */

/**
  * 
  * @param head ListNode类 
  * @return ListNode类
  */
function deleteDuplicates( head ) {
    // write code here
    let cache = new Set();
    let oldcache = new Set();
    while(head){
        
        if(oldcache.has(head.val)){
            cache.delete(head.val)
        }else{
            cache.add(head.val);
            oldcache.add(head.val)
        }
        
        head = head.next;
        
    }
    let temp = Array.from(cache);
    if(temp.length === 0){
        return 
    }
    let nodes = new ListNode(temp[0]);
    let onode = nodes
    for(let i = 1;i<temp.length;i++){
        onode.next = new ListNode(temp[i]);
       onode = onode.next
    }
    
    return nodes
}
function ListNode(x){
    this.val = x;
   this.next = null;
  }
module.exports = {
    deleteDuplicates : deleteDuplicates
};

第二种方法

 function deleteDuplicates( head ) {
    // write code here
    if(!head || !head.next){
        return head;
    }
    let res = new ListNode(-1); // 设置一个伪节点
    res.next = head;
    let cur = head;
    let prev = res;
    while(cur && cur.next){ // 如果有一个为null，说明已经到头了，不会再有重复
        if(cur.val == cur.next.val){ // 发现重复
            while(cur.val == cur.next.val){
                cur.next = cur.next.next;
                if(!cur.next){// 如果移除到底，则停止
                    break;
                }
            }
            prev.next = cur.next;
            cur = prev.next
        }else{
            prev = cur;
            cur = cur.next
        }
        
        
    }
    return res.next;
}