Leetcode-Sort List

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

/** * Definition for singly-linked list. * class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { *         val = x; *         next = null; *     } * } */public class Solution { //use the fast and slow pointer to get the middle of a ListNode       ListNode getMiddleOfList(ListNode head) {          ListNode slow = head;          ListNode fast = head;          while(fast.next!=null&&fast.next.next!=null) {              slow = slow.next;              fast = fast.next.next;          }          return slow;      }            public ListNode sortList(ListNode head) {          if(head==null||head.next==null) {              return head;          }          ListNode middle = getMiddleOfList(head);          ListNode next   = middle.next;              middle.next = null;          return mergeList(sortList(head), sortList(next));      }            //merge the two sorted list      ListNode mergeList(ListNode a, ListNode b) {          ListNode dummyHead = new ListNode(-1);          ListNode curr = dummyHead;          while(a!=null&&b!=null) {              if(a.val<=b.val) {                  curr.next=a;a=a.next;              } else {                  curr.next=b;b=b.next;              }              curr  = curr.next;          }          curr.next = a!=null?a:b;          return dummyHead.next;      }  }