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}.

分3步：

1.快慢指针找到中点（1-2-3-4-5-6）

2 反转后半部分（1-2-3-6-5-4）

3.merge （1-6-2-5-4-3-4）

 1 class Solution {
 2     public void reorderList(ListNode head) {
 3         if(head !=null){
 4             ListNode slower = head;
 5             ListNode faster = head;
 6             while(faster != null && faster.next != null){
 7                 faster = faster.next.next;
 8                 slower = slower.next;
 9             }
10             ListNode p2 = reverseList(slower);
11             ListNode p1 = head;
12             while(p2.next!=null){
13                 ListNode p1_next  = p1.next;
14                 ListNode p2_next =  p2.next;
15                 p2.next = p1.next;
16                 p1.next = p2;
17                 p2 = p2_next;
18                 p1 =p1_next;
19             }
20         }
21    
22         
23     }
24       public ListNode reverseList(ListNode head) {
25         if(head == null ||head.next == null) return head;
26         ListNode pre = head;
27         head = head.next;
28         pre.next = null;
29         while(head != null){
30             ListNode next = head.next;
31             head.next = pre;
32             pre = head;
33             head = next;
34         }
35         return pre;
36     }
37 }