咱这个空间是O1吧

class Solution {
    public ListNode sortList(ListNode head) {
        if(head==null || head.next==null) return head;
        //分成两段
        ListNode step1=head,step2=head.next;
        while(step2!=null && step2.next!=null){
            step1=step1.next;
            step2=step2.next.next;
        }
        ListNode h=one.next;
        one.next=null;
        ListNode sortedLeftHead=sortList(head);
        ListNode sortedRightHead=sortList(h);
        return merge(sortedLeftHead,sortedRightHead);
    }

    ListNode merge(ListNode l1,ListNode l2){
        if(l1==null) return l2;
        if(l2==null) return l1;
        if(l1.val<l2.val){
            l1.next=merge(l1.next,l2);
            return l1;
        }else{
            l2.next=merge(l1,l2.next);
            return l2;
        }
    }
}