143. 重排链表

给定链表 1->2->3->4, 重新排列为 1->4->2->3.

给定链表 1->2->3->4->5, 重新排列为 1->5->2->4->3.

class Solution {
    public ListNode reorderList(ListNode head) {
        if(head == null || head.next == null || head.next.next == null) return head;
        ListNode low = head;
        ListNode high = head;
        ListNode headPointer = head;
        Stack<ListNode> stack = new Stack<>();
        //快慢指针寻找中点
        while( high != null && high.next != null ){
            low = low.next;
            high = high.next.next;
        }
        //切分链表
        ListNode seconed = low.next;
        low.next = null;
        //用栈保存后一段链表，主要是翻转链表
        while(seconed != null){
            stack.push(seconed);
            seconed = seconed.next;
        }
        //弹栈 逐个插入第一个链表
        while(!stack.isEmpty()){
            ListNode insertNode = stack.pop();
            insertNode.next = headPointer.next;
            headPointer.next = insertNode;
            headPointer = headPointer.next.next;
        }
        return head;
    }
}