[C/C++] List相关操作

List相关操作

1 链表二分

目标：

（1）对于偶数节点，正好对半分；

（2）对于奇数节点，前 = 后 + 1

（3）断开链表，方便后期合并

cpp 复制代码

// 使用快慢指针完成中点拆分
ListNode *SplitList(ListNode *head) {
    ListNode *slow{head};
    ListNode *fast{head};

    while (fast->next != nullptr && fast->next->next != nullptr) {
        slow = slow->next;
        fast = fast->next->next;
    }

    ListNode *mid = slow->next;
    slow->next = nullptr;
    return mid;
}

2 链表合并

cpp 复制代码

ListNode *MergeList(ListNode *head1, ListNode *head2) {
    ListNode dummy{};
    ListNode *cur = &dummy;

    while (head1 != nullptr && head2 != nullptr) {
        if (head1->val < head2->val) {
            cur->next = head1;
            head1 = head1->next;
        } else {
            cur->next = head2;
            head2 = head2->next;
        }

        cur = cur->next;
    }


    cur->next = (head1 != nullptr) ? head1 : head2;
    return dummy.next;
}

3 链表排序

cpp 复制代码

ListNode* sortList(ListNode* head) {
    // 题目进阶要求 nlgn => 希尔/归并/快速/堆
    if (head == nullptr || head->next == nullptr) {
        return head;
    }

    ListNode *head2 = SplitList(head);

    head = sortList(head);
    head2 = sortList(head2);

    return MergeList(head, head2);
}

4 多链表合并

cpp 复制代码

ListNode* mergeKLists(vector<ListNode*>& lists) {
    auto cmp = [](const ListNode *lhs, const ListNode *rhs) {
        return lhs->val > rhs->val;
    };

	// 通过优先级队列进行排序，并将取出的后续节点继续插入
    priority_queue<ListNode *, vector<ListNode *>, decltype(cmp)> pq{};

    for (auto head : lists) {
        if (head != nullptr) {
            pq.push(head);
        }
    }

    ListNode dummy{};
    ListNode *cur = &dummy;

    while (!pq.empty()) {
        // 取最小
        ListNode *node = pq.top();
        pq.pop();

        if (node->next != nullptr) {
            pq.push(node->next);
        }

        cur->next = node;
        cur = node;
    }

    return dummy.next;
}