C++ 链表的归并排序

void disp_nodeList(ListNode *p) {    while (p!=NULL) {        disp(p->val);        p = p->next;    }}class Solution {public:    ListNode* sortList(ListNode* head) {        if (head==NULL || head->next==NULL) {            return head;        }        // step 1: divide into 2 parts        ListNode *slow = head;        ListNode *fast = head;        ListNode *pre = head;        while(fast!=NULL && fast->next!=NULL) {            pre = slow;            slow = slow->next;            fast = fast->next->next;        }        // step 2: cut list at middle        pre->next = NULL;        // step 3: merge 2 sublist;        return merge(sortList(head),sortList(slow));    }    ListNode *merge(ListNode *l1, ListNode *l2){        ListNode *p = NULL;        ListNode *head = NULL;        ListNode node(-1);        p = &node;        head = p;        while (l1!=NULL && l2!=NULL) {            if(l1->val < l2->val) {                p->next = l1;                l1 = l1->next;                p = p->next;            } else {                p->next = l2;                l2 = l2->next;                p = p->next;            }        }        if (l1!=NULL) {            p->next = l1;        } else{            p->next = l2;        }        return head->next;    }};int main() {    Solution solution;    ListNode l0(0);    ListNode l1(3);    ListNode l2(2);    ListNode l3(5);    ListNode l4(4);    ListNode l5(1);    l0.next = &l1;    l1.next = &l2;    l2.next = &l3;    l3.next = &l4;    l4.next = &l5;    ListNode *head = solution.sortList(&l0);    disp_nodeList(head);    return 0;}