Leetcode 148. sorting linked list

class Solution {
    
    public ListNode sortList(ListNode head) {
    
        return sortList(head,null);
    }
      public ListNode sortList(ListNode head, ListNode tail) {
    
        if (head == null) {
    
            return head;
        }
        if (head.next == tail) {
    
            head.next = null;
            return head;
        }
        ListNode slow = head, fast = head;
        while (fast != tail) {
    
            slow = slow.next;
            fast = fast.next;
            if (fast != tail) {
    
                fast = fast.next;
            }
        }
        ListNode mid = slow;
        ListNode list1 = sortList(head, mid);
        ListNode list2 = sortList(mid, tail);
        ListNode sorted = merge(list1, list2);
        return sorted;
    }
    public ListNode merge(ListNode head1,ListNode head2){
    
        ListNode dummy=new ListNode(0);
        ListNode temp = dummy, tmp1 = head1, tmp2 = head2;
        while(tmp1!=null&&tmp2!=null){
    
            if(tmp1.val<tmp2.val){
    
                temp.next=tmp1;
                tmp1=tmp1.next;
            }
            else{
    
                temp.next=tmp2;
                tmp2=tmp2.next;
            }
            temp=temp.next;
        }
        if(tmp1!=null) temp.next=tmp1;
        else if(tmp2!=null) temp.next=tmp2;
        return  dummy.next;
    }
}