143. Reorder List

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You must do this in-place without altering the nodes' values.

For example,
Given {1,2,3,4}, reorder it to {1,4,2,3}.

含义：按照指定规则，重排列列表

 

        if (head == null || head.next == null) return;
        ListNode walker = head, runner = head;
        while (runner != null && runner.next != null) {
            walker = walker.next;
            runner = runner.next.next;
        }
        walker = walker.next;
        Stack<Integer> values = new Stack<>();
        while (walker != null) {
            values.push(walker.val);
            walker = walker.next;
        }
        ListNode p = head;
        while (!values.isEmpty()) {
            ListNode newNode = new ListNode(values.pop());
            newNode.next = p.next;
            p.next = newNode;
            p = p.next.next;
        }
        if (runner == null) {
//            isEven = true;
            p.next.next = null;
        } else p.next = null;