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}.

就是一道快慢指针加上反转链表的题，仔细点就好。

写的有点啰嗦，还需要进一步优化下，里边的变量有点多。

代码：

public void reorderList(ListNode head) {        if(head == null || head.next == null) return;                ListNode slow = head;        ListNode fast = head;        while(fast.next != null && fast.next.next != null){            fast = fast.next.next;            slow = slow.next;        }                //reverse 2nd half        ListNode p2 = reverse(slow.next);        slow.next = null;        ListNode p1 = head;        ListNode dummy = new ListNode(-1);        dummy.next = head;        ListNode p = dummy;                while(p1 != null && p2 != null){            p.next = p1;            ListNode temp1 = p1.next;            ListNode temp2 = p2.next;            p.next.next = p2;            p = p.next.next;            p1 = temp1;            p2 = temp2;        }                if(p1 != null){            p.next = p1;        }    }        private ListNode reverse(ListNode slow){        ListNode dummy = new ListNode(-1);        dummy.next = slow;        ListNode pre = dummy;        ListNode cur = slow;        while(cur != null){            ListNode temp = cur.next;            cur.next = pre;            pre = cur;            cur = temp;        }        slow.next = null;        return pre;    }