我这代码有不懂的地方，能解释下不？

Question

 #include<iostream>
using namespace std;
struct list
{
    int data;   struct list *next;
};
class Stack
{
    struct list *ptrf,*ptrb;
public:
    Stack()
    {
        ptrf=ptrb=NULL;
    }
    void push(int x)
    {
        struct list *newnode=new struct list;
        newnode->data=x;
        newnode->next=NULL;
        if(ptrb==NULL)
            ptrf=ptrb=newnode;
        else
        {
            ptrb->next=newnode;
            ptrb=newnode;
        }
    }
    int pop()
    {
        //struct list *top;
        int value;
        value=ptrf->data;
        //top=ptrf;
        ptrf=ptrf->next;//因为19行代码说是NULL，那么这里的ptrf不就也为NULL了？
        //delete top;
        return value;
    }
};
void main()
{
    Stack A;  int i;
    int arr[]={5,2,8,1,4,3,9,7,6};
    cout<<"入队顺序:";
    for(i=0;i<9;i++)
    {
        cout<<arr[i]<<" ";
        A.push(arr[i]);
    }
    cout<<endl<<"出队顺序:";
    for(i=0;i<9;i++)
        cout<<A.pop()<<" ";
    cout<<endl;
    system("pause");
}

Answer 1