C语言动态链表程序,有错误

//题目：输入n个同学的成绩，并且排序
//问题1：排序完最后一个数据变成 0 了
//问题2：如果在12行后面加一行 L = head; 为什么就不出结果了？

#include<stdio.h>
#include<malloc.h>
#include<stdlib.h>
struct student
{
    int data;
    struct student *next;
}a;
struct student *p,*q,*head;
void f(struct student *L,int n)
{
    int t,i,j;
    p=L;
    q = p->next;
    for(i=0;i<n;i++)
    {
        for(j=0;j<n;j++)
        {
            if(p->data < q->data) 
            {
                t = p->data;
                p->data = q->data;
                q->data = t;
            }
            p = q;
            q = q->next;
        }
        p = L;
        q = p->next;
    }
}
int main()
{
    struct student *p1;
    p1=&a;
    int i,n,c=0;
    printf("请输入n:\n");
    scanf("%d",&n);
    q=&a;
    printf("\n");
    q->next=NULL;
    printf("请输入%d个同学的成绩：\n",n);
    for(i=0;i<n;i++)
    {
        p = (struct student*)malloc(sizeof(struct student));
        c++;
        if(c==1) head = p;
        scanf("%d",&p->data);
        q->next = p;
        q = q->next;
    }
    f(p1,n);
    printf("\n");
    for(i=0;i<n;i++)
    {
        printf("%d\n", head->data);
        head = head->next;
    }
    return 0;
}

#include<stdio.h>
#include<stdlib.h>

struct student
{
    int data;
    struct student *next;
} *p,*q,*head;

void f(struct student *L)
{
    int t;
    p = L;
    while(p)//
    {
        q = p->next;
        while(q)//
        {
            if(p->data < q->data) 
            {
                t = p->data;
                p->data = q->data;
                q->data = t;
            }
            q = q->next;
        }        
        p = p->next;
    }
}
int main()
{
    int i,n;
    printf("请输入n:\n");
    scanf("%d",&n);

    printf("请输入%d个同学的成绩：\n",n);
    head=p=q=NULL;
    while(n--)
    {
        p = (struct student*)malloc(sizeof(struct student));
        p->next=NULL;

        scanf("%d",&p->data);
        if(head==NULL)
            q = head = p;
        else
        {
            q->next = p;
            q = q->next;            
        }        
    }

    f(head);
    printf("\n");

    q=head;
    while(q)
    {
        printf("%d\n", q->data);
        q = q->next;
    }

    system("pause");
    return 0;
}