带头结点的循环双链表的查找序号为i的元素，查找第一个值为x的元素，插入第i个元素，删除第i个元素的程序，c++实现

以下是带头结点的循环双链表的C++实现，包括了查找序号为i的元素，查找第一个值为x的元素，插入第i个元素，删除第i个元素的操作：

#include <iostream>
using namespace std;

// 双链表结构体
struct ListNode {
    int val;
    ListNode *prev, *next;
    ListNode(int x) : val(x), prev(NULL), next(NULL) {}
};

// 带头结点的双链表类
class DoubleLinkedList {
private:
    ListNode *head;
    int size;
public:
    // 构造函数，初始化头结点
    DoubleLinkedList() {
        head = new ListNode(0);
        head->prev = head;
        head->next = head;
        size = 0;
    }

    // 获取链表长度
    int getSize() {
        return size;
    }

    // 查找序号为i的元素
    ListNode* get(int i) {
        if (i < 0 || i >= size) {
            return NULL;
        }
        ListNode *cur = head->next;
        for (int j = 0; j < i; j++) {
            cur = cur->next;
        }
        return cur;
    }

    // 查找第一个值为x的元素
    ListNode* search(int x) {
        ListNode *cur = head->next;
        while (cur != head) {
            if (cur->val == x) {
                return cur;
            }
            cur = cur->next;
        }
        return NULL;
    }

    // 在第i个位置插入元素
    bool insert(int i, int x) {
        if (i < 0 || i > size) {
            return false;
        }
        ListNode *cur = head->next;
        for (int j = 0; j < i; j++) {
            cur = cur->next;
        }
        ListNode *node = new ListNode(x);
        node->prev = cur->prev;
        node->next = cur;
        cur->prev->next = node;
        cur->prev = node;
        size++;
        return true;
    }

    // 删除第i个元素
    bool remove(int i) {
        if (i < 0 || i >= size) {
            return false;
        }
        ListNode *cur = head->next;
        for (int j = 0; j < i; j++) {
            cur = cur->next;
        }
        cur->prev->next = cur->next;
        cur->next->prev = cur->prev;
        delete cur;
        size--;
        return true;
    }
};

int main() {
    DoubleLinkedList list;
    list.insert(0, 1);
    list.insert(1, 2);
    list.insert(2, 3);
    list.insert(3, 4);
    list.insert(4, 5);

    ListNode *node = list.get(2);
    if (node != NULL) {
        cout << "Node at position 2: " << node->val << endl;
    }

    node = list.search(3);
    if (node != NULL) {
        cout << "Node with value 3 found at position: " << (node - list.get(0)) << endl;
    }

    list.insert(2, 6);
    cout << "List after insertion: ";
    for (int i = 0; i < list.getSize(); i++) {
        cout << list.get(i)->val << " ";
    }
    cout << endl;

    list.remove(3);
    cout << "List after deletion: ";
    for (int i = 0; i < list.getSize(); i++) {
        cout << list.get(i)->val << " ";
    }
    cout << endl;

    return 0;
}

需要注意的是，在实现查找序号为i的元素时，如果i超出了链表的范围，返回的是NULL；在实现查找第一个值为x的元素时，如果链表中不存在值为x的元素，也返回NULL。在插入和删除元素时，需要注意更新链表的大小。