归并排序（merge sort）

1.归

    ListNode* sortList(ListNode* head) {
        if (head == nullptr || head->next == nullptr) return head;
        // 1.将待排序序列分为两部分
        ListNode* pre = nullptr, *slow = head, *fast = head;
        while(fast != nullptr && fast->next != nullptr)
        {
            pre = slow;
            slow = slow->next;
            fast = fast->next->next;
        }
        pre->next = nullptr;
        // 2.对两个子部分再次进行划分
        ListNode* l1 = sortList(head);
        ListNode* l2 = sortList(slow);
        // 3.合并
        return merge(l1, l2);
    }

2.并

    ListNode* merge(ListNode* l1, ListNode* l2)
    {
        ListNode* p = nullptr;
        if(l1->val < l2->val)
        {
            p = l1;
            l1 = l1->next;
        }
        else
        {
            p = l2;
            l2 = l2->next;
        }
        ListNode* l = p;
        
        while(l1 != nullptr && l2 != nullptr)
        {
            if(l1->val < l2->val)
            {
                p->next = l1;
                l1 = l1->next;
            }
            else
            {
                p->next = l2;
                l2 = l2->next;
            }
            p = p->next;
        }
        if (l1 != nullptr)
            p->next = l1;
        if (l2 != nullptr)
            p->next = l2;
        return l;
    }