leetcode-Sort List

class Solution {
public:
    ListNode* sortList(ListNode* head) {
        if(head == NULL || head->next == NULL)
        {
            return head;
        }
        ListNode dummy(-1);
        dummy.next = head;
        ListNode *slow = &dummy;
        ListNode *fast = &dummy;
        while(fast != NULL && fast->next != NULL)
        {
            slow = slow->next;
            fast = fast->next->next;
        }

        ListNode *p1 = head;
        ListNode *p2 = slow->next;
        slow->next = NULL;
        ListNode *pp1 = sortList(p1);
        ListNode *pp2 = sortList(p2);
        return mergeList(pp1, pp2);
    }
private:
    ListNode *mergeList(ListNode *p1, ListNode *p2)
    {
        if(p1 == NULL && p2 == NULL)
        {
            return NULL;
        }
        else if(p1 == NULL)
        {
            return p2;
        }
        else if(p2 == NULL)
        {
            return p1;
        }
        else
        {
            ListNode dummy(-1);
            ListNode *cur = &dummy;
            while(p1 != NULL && p2 != NULL)
            {
                if(p1->val < p2->val)
                {
                    cur->next = p1;
                    p1 = p1->next;
                }
                else
                {
                    cur->next = p2;
                    p2 = p2->next;
                }
                cur = cur->next;
            }
            if(p1 != NULL)
            {
                cur->next = p1;
            }
            if(p2 != NULL)
            {
                cur->next = p2;
            }
            return dummy.next;
        }
    }
};