Leetcode: Insertion Sort List

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;
    }
};