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;
}