用递归的方式解决

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param head ListNode类 
     * @return ListNode类
     */
    public ListNode deleteDuplicates (ListNode head) {
        if(head == null){
            return null;
        }
    
        if(head.next == null){
            return head;
        }

        ListNode  current = deleteDuplicates(head.next);
        if(head.val == current.val){
            head.next = current.next;
        }
        return head;

    }
}