双链表基本操作

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看欧立奇的《程序员面试宝典》的双向链表部分，发现其中创建双向链表和删除链表中得某一点的程序存在问题，现将已经调试通过的程序贴在下面：
#include <iostream>using namespace std;#define  LEN  sizeof(DNode)typedef struct DoubleNode{         int data;         struct DoubleNode *next;         struct DoubleNode *pre;}DNode;DNode *create(){         DNode *head，*p_new， *p_old;         int cycle=1;         int n=0;         int data;         head = (DNode*)malloc(sizeof(DNode));         p_old = head;         while(cycle)         {                   cout<<"Please input the num"<<endl;                   cin>>data;                   if(data != 0)                   {                            n=n+1;                            if (n==1)                            {                                     p_old->data=data;                                     p_old->next=NULL;                                     p_old->pre=NULL;                            }                            else                            {                                     p_new = (DNode*)malloc(sizeof(DNode));                                     p_new->data =data;                                     p_old->next = p_new;                                     p_new->pre = p_old;                                     p_old = p_new;                            }                                    }                   else                   {                            cycle = 0;                   }                 }         p_old->next = NULL;                  head ->pre = NULL;         return head;}DNode *del(DNode *head,int num){         DNode *p;         p = (DNode*)malloc(sizeof(DNode));         p=head;         while(num != p->data && p->next !=NULL)         {                   p =p->next;         }         if(num == p->data)         {                   if(p == head)                   {                            free(head);                            p = p->next;                            head = p;                   }                   else if(p->next ==NULL)                   {                            p->pre->next=NULL;                            free(p);                   }                   else                    {                            p->pre->next = p->next;                            p->next->pre =p->pre;                            free(p);                   }         }         else         {                   cout<<"cann't find the num"<<endl;         }         return head;} DNode *insert(DNode *head,int num){         DNode *p,*p_new;         p=(DNode*)malloc(LEN);         p_new=(DNode*)malloc(LEN);         p_new->data = num;         p=head;         while(num > p->data && p->next !=NULL)         {                   p = p->next;         }         if(num <= p->data)         {                   if(head == p)                   {                            p_new->next =p;                            p->pre = p_new;                            head = p_new;                            head ->pre = NULL;                   }                   else                   {                            p->pre->next = p_new;                            p_new->next = p;                            p->pre = p_new;                            p_new->pre = p->pre;                   }         }         else          {                   p->next = p_new;                   p_new->pre = p;                   p_new->next = NULL;         }         return head;}void main(){         DNode *head;         head = (DNode*)malloc(LEN);         head =create();         int num;         num = head->next->next->pre->data;         //int num1=17;         //head=del(head,17);         //int num2=head->next->data;         head = insert(head ,21);         int num3=head->next->next->next->next->data;}