1.

#include<bits/stdc++.h>
using namespace std;

struct Node
{
    int val;
    Node *prev,*next;
    Node(): prev(NULL),next(NULL){}
    Node(int _val): val(_val),prev(NULL),next(NULL){}
};

void print(Node *head)
{
    for(auto p=head;p;p=p->next) 
      cout<<p->val<<' ';
        cout<<endl;
}

int main()
{
    Node *head=new Node(),*tail=new Node();//哨兵
    head->next=tail,tail->prev=head;
    print(head);

    auto a=new Node(1);
    a->next=head->next;
    a->prev=head;
    head->next->prev=a;
    head->next=a;


    print(head);
    return 0;
}

2.

#include<bits/stdc++.h>
using namespace std;

struct Node
{
    int val;
    Node *prev,*next;
    Node(): prev(NULL),next(NULL){}
    Node(int _val): val(_val),prev(NULL),next(NULL){}
};

void print(Node *head)
{
    for(auto p=head;p;p=p->next) 
      cout<<p->val<<' ';
        cout<<endl;
}

int main()
{
    Node *head=new Node(),*tail=new Node();//哨兵
    head->next=tail,tail->prev=head;
    print(head);

    auto a=new Node(1);
    a->next=head->next;
    a->prev=head;
    head->next->prev=a;
    head->next=a;

    auto b=new Node(2);
    b->next=head->next;
    b->prev=head;
    head->next->prev=b;
    head->next=b;

    print(head);
    return 0;
}

3.

#include<bits/stdc++.h>
using namespace std;

struct Node
{
    int val;
    Node *prev,*next;
    Node(): prev(NULL),next(NULL){}
    Node(int _val): val(_val),prev(NULL),next(NULL){}
};

void print(Node *head)
{
    for(auto p=head;p;p=p->next) 
      cout<<p->val<<' ';
        cout<<endl;
}

int main()
{
    Node *head=new Node(),*tail=new Node();//哨兵
    head->next=tail,tail->prev=head;
    print(head);

    auto a=new Node(1);
    a->next=head->next;
    a->prev=head;
    head->next->prev=a;
    head->next=a;

    auto b=new Node(2);
    b->next=a->next;
    b->prev=a;
    a->next->prev=b;
    a->next=b;

    print(head);
    return 0;
}

4.delete

#include<bits/stdc++.h>
using namespace std;

struct Node
{
    int val;
    Node *prev,*next;
    Node(): prev(NULL),next(NULL){}
    Node(int _val): val(_val),prev(NULL),next(NULL){}
};

void print(Node *head)
{
    for(auto p=head;p;p=p->next) 
      cout<<p->val<<' ';
        cout<<endl;
}

int main()
{
    Node *head=new Node(),*tail=new Node();//哨兵
    head->next=tail,tail->prev=head;
    print(head);

    auto a=new Node(1);
    a->next=head->next;
    a->prev=head;
    head->next->prev=a;
    head->next=a;

    auto b=new Node(2);
    b->next=a->next;
    b->prev=a;
    a->next->prev=b;
    a->next=b;

    print(head);

    //shanchu
    b->prev->next=b->next;
    b->next->prev=b->prev;
    print(head);
    return 0;
}

5.循环链表

#include<bits/stdc++.h>
using namespace std;

struct Node
{
    int val;
    Node *prev,*next;
    Node(): prev(NULL),next(NULL){}
    Node(int _val): val(_val),prev(NULL),next(NULL){}
};

void print(Node *head)
{
    for(auto p=head->next;p!=head;p=p->next) 
      cout<<p->val<<' ';
        cout<<endl;
}

int main()
{
    Node *head=new Node(),*tail=head;//哨兵
    head->next=tail,tail->prev=head;
    print(head);

    auto a=new Node(1);
    a->next=head->next;
    a->prev=head;
    head->next->prev=a;
    head->next=a;

    auto b=new Node(2);
    b->next=a->next;
    b->prev=a;
    a->next->prev=b;
    a->next=b;

    print(head);

    //shanchu
    b->prev->next=b->next;
    b->next->prev=b->prev;
    print(head);
    return 0;
}