Sort List

public class Solution {
    public ListNode sortList(ListNode head) {
        if(head == null) return null;
        if(head.next == null)   return head;
        ListNode mid = getMiddle(head);
        ListNode right = sortList(mid.next);
        mid.next = null;
        ListNode left = sortList(head);
        return mergeList(left,right);
    }
    public ListNode getMiddle(ListNode head)
    {
        ListNode slow = head;
        ListNode fast = head.next;
        while(fast != null && fast.next != null)
        {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }
    public ListNode mergeList(ListNode head1, ListNode head2)
    {
        ListNode dummy = new ListNode(-1);
        ListNode head = dummy;
        while(head1 != null && head2 != null)
        {
            if(head1.val < head2.val)
            {
                dummy.next = head1;
                head1 = head1.next;
            }
            else
            {
                dummy.next = head2;
                head2 = head2.next;
            }
            dummy = dummy.next;
        }
        if(head1 != null)   dummy.next = head1;
        if(head2 != null)   dummy.next = head2;
        return head.next;
    }
}

public ListNode getMiddle(ListNode head)
    {
        ListNode slow = head;
        ListNode fast = head.next;
        while(fast != null && fast.next != null)
        {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }