LeetCode-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}.

 1 /**
 2  * Definition for singly-linked list.
 3  * class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public void reorderList(ListNode head) {
14         if(head == null || head.next == null){
15             return;
16         }
17         ListNode mid = findMid(head);
18         ListNode head2 = reverseList(mid.next);
19         mid.next = null;
20         mergeTwoLists(head, head2);
21         
22         
23     }
24     
25     
26     public ListNode findMid(ListNode head){
27         ListNode fast = head.next;
28         ListNode slow = head;
29         while(fast != null && fast.next != null){
30             slow = slow.next;
31             fast = fast.next.next;
32         }
33         return slow;
34     }
35     
36     public ListNode reverseList(ListNode head){
37         ListNode nextNode = head.next;
38         head.next = null;
39         while(nextNode != null){
40             ListNode temp = nextNode.next;
41             nextNode.next = head;
42             head = nextNode;
43             nextNode = temp;
44         }
45         return head;
46     }
47     
48     public ListNode mergeTwoLists(ListNode head1, ListNode head2){
49         ListNode dummy = new ListNode(-1);
50         ListNode tail = dummy;
51         boolean isList1 = true;
52         while(head1 != null && head2 != null){
53             if(isList1){
54                 tail.next = head1;
55                 head1 = head1.next;
56                 isList1 = false;
57             }
58             else if(!isList1){
59                 tail.next = head2;
60                 head2 = head2.next;
61                 isList1 = true;
62             }
63             tail=tail.next;
64         }
65         while(head1 != null){
66             tail.next = head1;
67             head1 = head1.next;
68             tail =tail.next;
69         }
70         
71         while(head2 != null){
72             tail.next = head2;
73             head2 = head2.next;
74             tail = tail.next;
75         }
76         
77         return dummy.next;
78         
79     }
80     
81     
82 }