143. Reorder List

Example 1:

Given 1->2->3->4, reorder it to 1->4->2->3.
Example 2:

Given 1->2->3->4->5, reorder it to 1->5->2->4->3.

idea:
（自己想的）
iteration: hashmap存每个node的previous node。两个指针一个从前一个从最后开始，不断把前指针的next设置成后一个指针对应的元素，直到head == tail or head.next == tail

code：

        while (head != tail && head.next != tail) {
            ListNode storeHead = head.next;
            head.next = tail;
            ListNode storeTail = map.get(tail);
            storeTail.next = null;
            tail.next = storeHead;
            
            head = storeHead;
            tail = storeTail;
        }