Leetcode148 sort linked list (merge method applied to merge)

Here's the head of the list  head , Please sort them in ascending order and return the sorted list

/**
 *  Using merge sort merge Method ： First, disassemble the big list into a small list , Then combine the small linked list back to an ascending large linked list 
 *  Find the middle point of the linked list when splitting ： Speed pointer 
 *  Merge two ascending linked lists ： see leetcode21
 */
public class Num148_SortList {
    private static class ListNode {
      int val;
     ListNode next;
      ListNode() {}
      ListNode(int val) { this.val = val; }
      ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 }
    public static ListNode sortList(ListNode head) {
        return sortListHelper(head,null);
    }
    // Split the list 
    private static ListNode sortListHelper(ListNode head, ListNode tail) {
        if(head==null){
            return head;
        }
        if(head.next==tail){
            head.next=null;
            return head;
        }
        // Find the middle point of the linked list ： Speed pointer 
        ListNode slow=head;
        ListNode fast=head;
        while (fast!=tail){
            fast=fast.next;
            slow=slow.next;
            if(fast!=tail){
                fast=fast.next;
            }
        }
        // At this time, the intermediate node of the linked list is found 
        ListNode mid=slow;
        // Split the left linked list 
        ListNode leftList=sortListHelper(head,mid);
        // Split the right linked list 
        ListNode rightList=sortListHelper(mid,tail);
        // At this time, the linked list is split , Conduct merge Operation merge linked list 
        return merge(leftList,rightList);
    }

    private static ListNode merge(ListNode leftlist, ListNode rightlist) {
        // Merge two linked lists （ Ascending ）; Return the merged linked list 
        if(leftlist==null){
            return leftlist;
        }
        if(rightlist==null){
            return rightlist;
        }
        // At this time, both linked lists are not empty , Merge two linked lists 
        // Create a virtual head node , Insert with tail 
        ListNode dummyHead=new ListNode(10*10*10*10*10+1);
        ListNode cur=dummyHead;
        while (leftlist!=null&&rightlist!=null){
            if(leftlist.val<=rightlist.val){
                cur.next=leftlist;
                cur=leftlist;
                leftlist=leftlist.next;
            }else{
                cur.next=rightlist;
                cur=rightlist;
                rightlist=rightlist.next;
            }
            // Judge left And right Whether to traverse to null 
            if(leftlist==null){
                cur.next=rightlist;
            }
            if(rightlist==null){
                cur.next=leftlist;
            }
        }
        return dummyHead.next;
    }

    public static void main(String[] args) {
        ListNode node1=new ListNode(-1);
        ListNode node2=new ListNode(5);
        ListNode node3=new ListNode(3);
        ListNode node4=new ListNode(4);
        ListNode node5=new ListNode(0);
        node1.next=node2;
        node2.next=node3;
        node3.next=node4;
        node4.next=node5;
        ListNode res=sortList(node1);
        System.out.println(res);
    }
}

Related issues ：

Power button 147- Insert and sort the linked list