sort list merge sort recursive

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *sortList(ListNode *head) {
        if(head==NULL || head->next==NULL)
            return head;
        ListNode *p=head,*q=head;
        while(p->next!=NULL && p->next->next!=NULL){
            p=p->next->next;
            q=q->next;
        }
        p=q;
        q=q->next;
        p->next=NULL;
        p=sortList(head);
        q=sortList(q);
        return merge(p,q);
    }
    ListNode *merge(ListNode*p,ListNode*q){
        ListNode *h=new ListNode(0),*c=h;
        while(p!=NULL && q!=NULL){
            if(p->val<q->val){
                c->next=p;
                p=p->next;
                c=c->next;
            }else{
                c->next=q;
                q=q->next;
                c=c->next;
            }
        }
        if(p!=NULL)
            c->next=p;
        if(q!=NULL)
            c->next=q;
        c=h->next;
        delete h;
        return c;
    }
   
};
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *sortList(ListNode *head) {
        if(head==NULL || head->next==NULL)
            return head;
        ListNode *p=head,*q=head;
        while(p->next!=NULL && p->next->next!=NULL){
            p=p->next->next;
            q=q->next;
        }
        p=q;
        q=q->next;
        p->next=NULL;
        p=sortList(head);
        q=sortList(q);
        return merge(p,q);
    }
    ListNode *merge(ListNode*p,ListNode*q){
        ListNode *h=new ListNode(0),*c=h;
        while(p!=NULL && q!=NULL){
            if(p->val<q->val){
                c->next=p;
                p=p->next;
                c=c->next;
            }else{
                c->next=q;
                q=q->next;
                c=c->next;
            }
        }
        if(p!=NULL)
            c->next=p;
        if(q!=NULL)
            c->next=q;
        c=h->next;
        delete h;
        return c;
    }
   
};