C++双相链表，插入元素，但是总不成功，请大神帮忙找出错误

Question

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

Answer 1

1.插入位置是 0的时候需要单独考虑

2.后面插入的逻辑不对，应该是

nide*  t = p->pre;

t->next = s; 

s->pre = t;

s->next = p;

p->pre = s;