首先定义一个节点类：

class ListNode {
     int val;
     ListNode next;
     ListNode(int x) {
         val = x;
         next = null;
     }
     public int getVal(){return val;}
     public void setVal(int val){this.val = val;}
     public ListNode getNext(){return next;}
     public void setNext(ListNode next){this.next = next;}
 }

然后初始化两个链表：｛1,2,4｝，｛1,3,4｝

public class justForTest7 {
    public static void main(String[] args) {
        ListNode head = new ListNode(1);
        ListNode head1 = new ListNode(2);
        ListNode head2 = new ListNode(4);
        head.setNext(head1);
        head1.setNext(head2);
        ListNode preNode = new ListNode(1);
        ListNode preNode1 = new ListNode(3);
        ListNode preNode2 = new ListNode(4);
        preNode.setNext(preNode1);
        preNode1.setNext(preNode2);
//关键代码
        ListNode first = head;      //first指向第一个链表的头结点
        ListNode after = preNode;   //after指向第二个链表的头结点
        while (first != null && after != null) {
            ListNode firstTemp = first.next;
            ListNode afterTemp = after.next;
            first.next = after;     //将after的第一个数放在first后面
            first = firstTemp;      //first后移到firstTemp
            after.next = first;     //after指向新的first
            after = afterTemp;      //after后移到afterTemp
        }
         
        ListNode testNode = head;
        while (testNode != null)
        {
            System.out.print(testNode.val);
            testNode = testNode.next;
        }
    }
}

运行结果：