```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 fast = head;
        while(fast.next != null) {
            if (fast.val == fast.next.val) {
                fast.next = fast.next.next;
            } else {
                fast = fast.next;
            }
        }
        return head;
    }
}