Problem Reorder List

Problem Description:

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}.

 

Solution: 

public void reorderList(ListNode head) {
        int length = 0;
        ListNode p = head;
        List<ListNode> list = new ArrayList<ListNode>();
        while (p != null) {
            length++;
            list.add(p);
            p = p.next;
        }
        int i = 0;
        p = null;
        while (i <= (length - 1 - i)) {
            if (p != null) {
                p.next = list.get(i);
            }
            if (i != length -1 -i) {
                list.get(i).next = list.get(length - 1-i);
                p = list.get(length-1-i);
                p.next = null;
            } else {
                list.get(i).next = null;
            }

            i++;
        }
    }