解法1：

class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if(null==head){
            return null;
        }
        ListNode phead=head;
        while(null!=phead.next){
            if(phead.next.val==phead.val){
                phead.next=phead.next.next;
            }else{
                phead=phead.next;
            }
        }
        return head;

    }
}

解法2：递归

class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if(null==head ||null ==head.next){
            return head;
        }   
        head.next=deleteDuplicates(head.next);  
        return head.val==head.next.val?head.next:head;

    }
}