C++归并法+快速排序实现链表排序的方法

class Solution {
public:
    ListNode* sortList(ListNode* head) {
        return sortList(head, nullptr);
    }

    ListNode* mergesort(ListNode* head, ListNode* tail) {
        if (head == nullptr) {
            return head;
        }
        if (head->next == tail) {
            head->next = nullptr;
            return head;
        }
        ListNode* slow = head, * fast = head;
        while (fast != tail) {
            slow = slow->next;
            fast = fast->next;
            if (fast != tail) {
                fast = fast->next;
            }
        }
 
        return merge( mergesort(head, slow),  mergesort(slow, tail));
    }

    ListNode* merge(ListNode* head1, ListNode* head2) {
        ListNode* dummyHead = new ListNode(0);
        ListNode* temp = dummyHead, * temp1 = head1, * temp2 = head2;
        while (temp1 != nullptr && temp2 != nullptr) {
            if (temp1->val <= temp2->val) {
                temp->next = temp1;
                temp1 = temp1->next;
            }
            else {
                temp->next = temp2;
                temp2 = temp2->next;
            }
            temp = temp->next;
        }
        if (temp1 != nullptr) {
            temp->next = temp1;
        }
        else if (temp2 != nullptr) {
            temp->next = temp2;
        }
        return dummyHead->next;
    }
};

class Solution {
    public static ListNode sortList(ListNode head) {
        return quickSort(head ,null);
    }

    public static ListNode quickSort(ListNode head ,ListNode end){
        if(head ==end || head.next ==end) return head;
        ListNode lhead = head ,utail = head ,p = head.next;
        while (p != end){
            ListNode next = p.next;
            if(p.val < head.val){//头插
                p.next = lhead;
                lhead = p;
            }
            else { //尾插
                utail.next = p;
                utail = p;
            }
            p = next;
        }
        utail.next = end;
        ListNode node = quickSort(lhead, head);
        head.next =  quickSort(head.next, end);
        return node;
    }
}