[Bug] 两个链表，交错组合为另一个链表

ListNode dummy3 = new ListNode(-1);
ListNode pre = dummy3;
while(head2!=null&&head1!=null){
     pre.next=head1;
     ListNode head1next = head1.next;
     head1.next=head2;
     ListNode head2next = head2.next;
     head2.next=null;
     pre=head2;
     head1=head1next;
     head2=head2next;
 }

 1 ListNode dummy3 = new ListNode(-1);
 2 ListNode pre = dummy3;
 3 while(head2!=null&&head1!=null){
 4      pre.next=head1;
 5      pre.next.next = head2;
 6      //ListNode head1next = head1.next;
 7      //head1.next=head2;
 8      //ListNode head2next = head2.next;
 9      //head2.next=null;
10      pre=head2;
11      head1=head.next;
12      head2=head.next;
13  }