题解 | #牛群排列去重#

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {

    public ListNode deleteDuplicates (ListNode head) {
        if(head==null) return head;
        ListNode cur=head.next;
        ListNode pre=head;

        Set<Integer> set = new HashSet<>();   // 初始化一个 HashSet
        set.add(head.val);
        while(cur!=null){
            if(set.contains(cur.val)){
                pre.next=cur.next;
                cur=cur.next;
                continue;
            }
            set.add(cur.val);
            pre=cur;
            cur=cur.next;
        }
        return head;
    }
}