Reorder List

 1 public class Solution {
 2     public void reorderList(ListNode head) 
 3     {
 4         if(head==null||head.next==null) return;
 5         
 6         ListNode slow = head;
 7         ListNode fast = head;
 8         ListNode firsthalf = head;
 9         
10         while(fast!=null&&fast.next!=null&&fast.next.next!=null)
11         {
12             slow = slow.next;
13             fast = fast.next.next;
14         }
15         
16         ListNode secondhalf = slow.next;
17         slow.next = null;
18         
19         secondhalf = reverseList(secondhalf);
20         ListNode result = new ListNode(0);
21         
22         boolean flag = false;
23        /* while (secondhalf.next!=null&firsthalf.next!=null)
24         {
25         if (flag == false)
26         {
27             result.next=firsthalf;
28             firsthalf=firsthalf.next;
29             flag=true;
30         }
31         if(flag == true)
32         {
33             result.next=secondhalf;
34             secondhalf=secondhalf.next;
35             flag=false;
36         }
37         } */
38         
39         while (secondhalf!= null&&firsthalf!=null) 
40         {
41             ListNode temp1 = firsthalf.next;
42             ListNode temp2 = secondhalf.next;
43  
44             firsthalf.next = secondhalf;
45             secondhalf.next = temp1;        
46  
47             firsthalf  = temp1;
48             secondhalf = temp2;
49         }
50         
51     }
52     
53     public ListNode reverseList(ListNode head) 
54     {
55         if (head == null) return head;
56         ListNode cur = head;
57         ListNode pre = null;
58         while (cur.next != null){
59             ListNode nextNode = cur.next;
60             cur.next = pre;
61             pre = cur;
62             cur = nextNode;
63         }
64         cur.next = pre;
65         return cur;
66     }
67 }