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 || head.next == null) {
            return head;
        }
        ListNode tail = head;
        ListNode cur = head.next;
        while (cur != null) {
            if (cur.val != tail.val) {
                tail.next = cur;
                tail = cur;
            }
            cur = cur.next;
        }
        tail.next = null;
        return head;
    }
}