双向链表的三种插入方法研究

struct node_each node{
　　int data;
　　struct node_each *prev;
　　struct node_each *next;
}; 

{
    D->next = head; //M2
    head->prev = D; //M1
    head = D;
}

{
　　//body -> C
    if (body->next == NULL)
    {
        body->next = D; //M1
        D->prev = body; //M2
    }
}

{
　　if (body->next == NULL)
　　{
    　　//print nothing
　　}
　　else
　　{
    //A as body after D
    　　body->next-prev = D; //M2
    　　D->next = body->next; //M1
    　　body->next = D; //M4
    　　D->prev = body;//M3
　　}
}