text:  var XXX = function(head) {
        if(!head){return head;}  //尾部返回
        while(head.next&&head.val === head.next.val){
              head.next=head.next.next;    //   当前值跟下一个值 一样时， 跳过当前值
        }
       // console.log(head.val);
      XXX(head.next); //依次递归
      return head
      
  };
  
  