在java中删除链表中的重复项

import java.util.HashSet;

class ListNode {
    int val;
    ListNode next;

    ListNode(int val) {
        this.val = val;
    }
}

public class Main {
    public static ListNode deleteDuplicates(ListNode head) {
        if (head == null) {
            return null;
        }
        
        HashSet<Integer> set = new HashSet<>();
        ListNode current = head;
        ListNode prev = null;
        
        while (current != null) {
            if (set.contains(current.val)) {
                prev.next = current.next;
            } else {
                set.add(current.val);
                prev = current;
            }
            current = current.next;
        }
        
        return head;
    }
    
    public static void main(String[] args) {
        // 创建一个有重复项的链表: 1 -> 2 -> 2 -> 3 -> 3 -> 4
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(2);
        head.next.next.next = new ListNode(3);
        head.next.next.next.next = new ListNode(3);
        head.next.next.next.next.next = new ListNode(4);
        
        // 删除重复项
        head = deleteDuplicates(head);
        
        // 输出结果：1 -> 2 -> 3 -> 4
        ListNode current = head;
        while (current != null) {
            System.out.print(current.val + " -> ");
            current = current.next;
        }
        System.out.println("null");
    }
}