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;
}
};