gold-2-1

编写代码，移除未排序链表中的重复节点。保留最开始出现的节点。

示例1:

输入：[1, 2, 3, 3, 2, 1]
输出：[1, 2, 3]

示例2:

输入：[1, 1, 1, 1, 2]
输出：[1, 2]

提示：

1. 链表长度在[0, 20000]范围内。
2. 链表元素在[0, 20000]范围内。

进阶：

如果不得使用临时缓冲区，该怎么解决？

//leetcode submit region begin(Prohibit modification and deletion)
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode removeDuplicateNodes(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }

        ListNode pre = head;
        ListNode cur = pre.next;
        Set<Integer> mySet = new HashSet<>();
        mySet.add(pre.val);

        while (cur != null) {
            if (mySet.contains(cur.val)) {
                cur = cur.next;
            } else {
                mySet.add(cur.val);
                pre.next = cur;
                pre = cur;
                cur = cur.next;
            }
        }

        pre.next = null;
        return head;
    }
}
//leetcode submit region end(Prohibit modification and deletion)