Sort a linked list in O(n log n) time using constant space complexity.

 1 public class Solution {
 2     public ListNode sortList(ListNode head) {
 3         if (head == null || head.next == null) return head;
 4         return mergesort(head);
 5     }
 6
 7     public ListNode mergesort(ListNode head) {
 8         if (head == null || head.next == null) return head;
 9         ListNode dummy = new ListNode(-1);
10         dummy.next = head;
11         ListNode walker = dummy;
12         ListNode runner = dummy;
13         while (runner!=null && runner.next!=null) {
14             runner = runner.next.next;
15             walker = walker.next;
16         }
17         ListNode head1 = dummy.next;
18         ListNode head2 = walker.next;
19         walker.next = null;
20         return merge(mergesort(head1), mergesort(head2));
21     }
22
23     public ListNode merge(ListNode head1, ListNode head2) {
24         if (head1 == null) return head2;
25         if (head2 == null) return head1;
26         ListNode dummy = new ListNode(-1);
27         dummy.next = head1;
28         ListNode pre = dummy;
29         while (head1 != null && head2 != null) {
30             if (head1.val <= head2.val) {
31                 head1 = head1.next;
32             }
33             else {
34                 ListNode next = head2.next;
35                 head2.next = pre.next;
36                 pre.next = head2;
37                 head2 = next;
38             }
39             pre = pre.next;
40         }
41         if (head2 != null) {
42             pre.next = head2;
43         }
44         return dummy.next;
45     }
46 }