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) {
        // write code here
        if(head==null)
            return null;
        ListNode p1 = head;
        ListNode p2 = head.next;
        while(p2!=null){
            if(p1.val==p2.val){
                p1.next=p2.next;
                p2 = p2.next;
            }else{
                p1 = p2;
                p2 = p2.next;
            }
        }
        return head;
    }
}