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;
}
