leetcode-005 reorder list

 1 package leetcode;
 2
 3 public class ReOrderList {
 4     public void reorderList(ListNode head) {
 5         if(head==null||head.next==null||head.next.next==null){
 6
 7         }else{
 8             int l=numNode(head);
 9             ListNode mid = new ListNode(-1);
10             mid=getMid(head);
11             ListNode next=mid.next;
12             ListNode po=reverse(next);
13             mid.next=null;
14             ListNode p=head;
15             while(po!=null){
16                 ListNode po2=po.next;
17                 po.next=p.next;
18                 p.next=po;
19                 p=p.next.next;
20                 po=po2;
21             }
22         }
23
24     }
25     public int numNode(ListNode head){
26         if(head==null){
27             return 0;
28         }
29         int i=0;
30         while(head!=null){
31             i++;
32             head=head.next;
33         }
34         return i;
35     }
36     public ListNode getMid(ListNode head){
37         ListNode p=head;
38         ListNode q=head;
39         ListNode pre=head;
40         while(q.next!=null&&q.next.next!=null){
41             pre=p;
42             p=p.next;
43             q=q.next.next;
44         }
45         return p;
46     }
47     public ListNode reverse(ListNode n){
48         ListNode h = new ListNode(-1);
49         ListNode q = n;
50         while(q!=null){
51             ListNode next=q.next;
52             q.next=h.next;
53             h.next=q;
54             q=next;
55         }
56         return h.next;
57     }
58     public static void main(String[] args){
59         ListNode a=new ListNode(1);
60         ListNode b=new ListNode(2);
61         ListNode c=new ListNode(3);
62         ListNode d=new ListNode(4);
63         a.next=b;
64         b.next=c;
65         c.next=d;
66         d.next=null;
67         ReOrderList s = new ReOrderList();
68         s.reorderList(a);
69         ListNode p=a;
70         while (p != null) {
71             System.out.println(p.val);
72             p = p.next;
73         }
74     }
75
76 }

leetcode-005 reorder list