class Solution {public:    ListNode *insertionSortList(ListNode *head) {        if(head == NULL || head->next == NULL) return head;        ListNode dummy(-1);        dummy.next = head;                for(ListNode *prevp = head, *p = head->next; p; p = prevp->next){            //it‘s worth noting that q starts from dummy.next not head            ListNode *prevq = &dummy, *q = dummy.next;            while( q != p && q->val <= p->val){                 prevq = q;                q = q->next;            }            if(q != p){                prevp->next = p->next;                p->next = q;                prevq->next = p;            }else{                prevp = p;            }        }        return dummy.next;    }};