双链表插入节点问题，麻烦各位给看下这个程序哪里有问题，谢谢

#include<iostream>
using namespace std;
struct node{
  int data;
  node *pre,*next;
};
node *head,*p,*r,*a;
void insert(int x,int y)
{
  int z=1;
  a=new node;
  a->data=x;
  p=head->next;
  while(p->next!=NULL&&z<y )
  {
      p=p->next;
      z++;
  }
  if(p==NULL) cout<<"no this position"<<endl;
  else
  {
      a->pre=p->pre;
      p->pre=a;
      a->next=p;
      p->pre->next=a;
  }
}
int main()
{
  int x,a,b;
  cin>>x;
  head=new node;
  r=head;
  while(x!=-1)
  {
      p=new node;
      p->data=x;
      p->next=NULL;
      p->pre=NULL;
      r->next=p;
      if(r!=head) p->pre=r;
      r=p;
      cin>>x;
  }
  cin>>a>>b;
  insert(a,b);
  p=head->next;  
  while(p->next!=NULL)
  {
      cout<<p->data<<" ";
      p=p->next;
  }
  cout<<p->data<<endl;
  system("pause");
  return 0;
}  

输入1 2 3 4 5 6 -1 5 3
输出1 2 3 4 5 6
把输出部分改成如下：
p=r;  
  while(p->pre!=NULL)
  {
      cout<<p->data<<" ";
      p=p->pre;
  }
  cout<<p->data<<endl;
输出为6 5 4 3 5 2 1
不明白是为什么，麻烦各位了，谢谢