链表死循环问题

#include "stdio.h"
#include "conio.h"
#include "stdlib.h"
struct xue
{
    int num;
    char name[20];
    float score;
    struct xue *next;
};
//建立链表
struct xue * LinkList()   
{
    struct xue *h,*a,*b;
    int i=0;
    printf("请输入学号,姓名,成绩\n");
    if((a=b=(struct xue*)malloc(sizeof(struct xue)))==NULL)
        printf("内存不足");
    h=NULL;
    scanf("%d%s%f",&a->num,a->name,&a->score);
    while(a->num!=0)
    {
        i++;
        if(i==1)
            h=a;
        else b->next=a;
        b=a;
        if((a=(struct xue*)malloc(sizeof(struct xue)))==NULL)
        printf("内存不足");        
        scanf("%d%s%f",&a->num,a->name,&a->score);
    }
    b->next=NULL;
    return h;
}

//输出链表
int print(struct xue *h1)
{
    struct xue *k;
    k=h1;
    if(k!=NULL)
    {
        printf("学号    名字    成绩\n");   
        while(k!=NULL)
        {
            printf("%d\t%s\t%.2f\n",k->num,k->name,k->score);
            k=k->next;
        }
    }
        printf("\n");
    return 0;
}
//删除链表
struct xue * Del_LinkList(struct xue * s, int n)
{
    struct xue *i,*j;
    i=s;
    if(i==NULL)
    {
        printf("错误!该表是空的\n");
        return s;
    }
        while(n!=i->num&&i!=NULL);
        {
            j=i;            
            i=i->next;
        }
            if(i->num==n)
                {
                    if(s==i)
                    {
                        free (s);                    
                        return i->next;
                    }
                    else j->next=i->next;
                }
            else printf("找不到该结点");
    return s;
}

int main()
{
    struct xue *head;
    int n;
    head=LinkList();
    print(head);
    printf("请输入你要删除的学号:");
    scanf("%d",&n);
    Del_LinkList(head,n);//输入学号后...CPU使用就100%了.可能这个函数有死循环..找了好久也没有找到问题
    print(head);
    getch();
    return 0;
}