请用java代码实现原有链表1-2-3-4 需要生成新的链表1-2-3

可以使用一个循环遍历原有链表，每次将当前节点的值添加到新的链表中，直到遍历到第三个节点为止。以下是一种可能的实现方式：

class ListNode {
    int val;
    ListNode next;

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

public class Main {
    public static void main(String[] args) {
        // 创建原有链表: 1->2->3->4
        ListNode head = new ListNode(1);
        ListNode node1 = new ListNode(2);
        ListNode node2 = new ListNode(3);
        ListNode node3 = new ListNode(4);
        head.next = node1;
        node1.next = node2;
        node2.next = node3;

        // 生成新的链表: 1->2->3
        ListNode newHead = generateNewList(head);

        // 打印新的链表
        ListNode currentNode = newHead;
        while (currentNode != null) {
            System.out.print(currentNode.val + " -> ");
            currentNode = currentNode.next;
        }
        System.out.println("null");
    }

    public static ListNode generateNewList(ListNode head) {
        ListNode newHead = new ListNode(head.val);
        ListNode currentNode = newHead;
        ListNode originalNode = head.next;
        int count = 1;
        while (originalNode != null && count < 3) {
            ListNode newNode = new ListNode(originalNode.val);
            currentNode.next = newNode;
            currentNode = newNode;
            originalNode = originalNode.next;
            count++;
        }
        return newHead;
    }
}

运行结果：

1 -> 2 -> 3 -> null