LeetCode 148. Sort List

class Solution {
public:
    ListNode* sortList(ListNode* head) {
        if (head==NULL || head->next==NULL) return head;
        ListNode *slow=head, *fast=head->next;
        while (fast!=NULL && fast->next!=NULL){
            slow = slow->next;
            fast = fast->next->next;
        }
        fast = slow->next;
        slow->next = NULL;
        return merge(sortList(head), sortList(fast));
    }
    
    ListNode *merge(ListNode *left, ListNode *right){
        ListNode *dummy=new ListNode(0);
        ListNode *p=left, *q=right, *r=dummy;
        while (p!=NULL && q!=NULL){
            if (p->val<q->val){r->next=p; p=p->next;}
            else {r->next=q; q=q->next;}
            r = r->next;
        }
        while (p!=NULL){r->next=p; p=p->next; r=r->next;}
        while (q!=NULL){r->next=q; q=q->next; r=r->next;}
        return dummy->next;
    }
};