Why fast is not pointing to 6. but it is pointing to 5

#include
using namespace std;
class node{
public:
int data;
nodenext;
node(int d)
{
data=d;
next=NULL;
}
};
/int length(nodehead)
{
int len=0;
while(head!=NULL)
{
head=head->next;
len+=1;
}
return len;
}
/
void reverse(node*&head)
{
nodec=head;
node
p=NULL;
noden;
while(c!=NULL)
{
n=c->next;
c->next=p;
p=c;
c=n;
}
head=p;
}
node
lastelement(nodehead,int pos)
{
if(head==NULL || head->next==NULL)
{
return head;
}
node
fast=head;
int jump=1;
while(jump<pos)
{
fast=fast->next;
jump++;
}
/while(fast!=NULL)
{
fast=fast->next;
slow=slow->next;
}
/
return fast;
}
void insertattail(node*&head,int data)
{
if(head==NULL)
{
head=new node(data);
return;
}
nodetail=head;
while(tail->next!=NULL)
{
tail=tail->next;
}
tail->next=new node(data);
return;
}
void print(node
head)
{
while(head!=NULL)
{
cout<data;
head=head->next;
}
}
void buildlist(node*&head)
{
int data;
cin>>data;
while(data!=-1)
{
insertattail(head,data);
cin>>data;
}
}
int main() {
nodehead=NULL;
buildlist(head);
reverse(head);
int pos;
cin>>pos;
node
m=lastelement(head,pos);
cout<data;
return 0;
}

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the code provided is passing all the test cases, can you elaborate the problem you are facing.

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