求大神看看哪里错了，运行中断

#include <iostream>
#include <stdlib.h>
using namespace std;
class score
{    public:
     int num;
     double Math;
     double English;
     double Programming;
     void inscore(score *head);
     void showscore(score *head);
     double average();
     score *next;
};
void score::inscore(score *head)
{   score *p=0,*q;
    q=new score;
    cout<<"输入学生的学号、数学、英语及程序设计成绩："<<endl;
    cin>>q->num>>q->Math>>q->English>>q->Programming;
    while( q->num!=0 && q->Math!=0 && q->English!=0 && q->Programming!=0 )
    { if(head==NULL) p=head=q;
      else p->next=q;
      p=q;
      q=new score;
      cout<<"输入学生的学号、数学、英语及程序设计成绩："<<endl;
      cin>>q->num>>q->Math>>q->English>>q->Programming;
    }
    q->next=NULL;
}
void score::showscore(score *head)
{   cout<<"学号    数学    英语    程序设计    平均成绩:"<<endl;
    while(head)
    {  cout<<head->num<<'\t'<<head->Math<<'\t'<<head->English<<'\t'<<head->Programming<<'\t'<<average()<<endl;
       head=head->next;
    }
    cout<<endl;
}
double score::average()
{   return (Math+English+Programming)/3;
}
int main()
{    score *head=NULL;
     head->inscore(head);
     head->showscore(head);
     return 0;
     system("pause");
}
求改正，最好讲解一下，谢谢

[ 本帖最后由 APTX 于 2015-5-22 11:57 编辑 ]