ListNode join(ListNode a, ListNode b) { ListNode res = new ListNode(0); ListNode cur = res; while (a != null && b != null) { cur.next = a; cur.next.next = b; cur = cur.next.next; a = a.next; b = b.next; } if (a == null) { while (b != null) { cur.next = b; cur = cur.next; b = b.next; } } else { while (a != null) { cur.next = a; cur = cur.next; a = a.next; } } return res.next; }

The function join takes two arguments a and b. It returns the listNode containing a and b.