Reorder List

三步走： 第一步，分成两部分；第二步，将第二部分reverse；第三步，将两个list merge起来。
一定要舍得画图，把指针的指向搞清楚。

public class Solution {
    public void reorderList(ListNode head) {
        if(head == null) return;
        ListNode slow = head, fast = head;
        while(fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        ListNode second = slow.next;
        if(second == null) return;
        slow.next = null;
        ListNode prev = second, curr = second.next;
        while(curr != null) {
            ListNode temp = curr.next;
            curr.next = prev;
            prev = curr;
            curr = temp;
        }
        second.next = null;
        second = prev;
        ListNode first = head;
        while(first != null && second != null) {
            ListNode secondNext = second.next;
            second.next = first.next;
            first.next = second;
            first = second.next;
            second = secondNext;
        }
    }
}