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

struct st
{
    int a;
    struct st * pNext;
};


struct st * create(void);
void show(struct st *);

int main()
{
    struct st * pHead;
    pHead=create();
    show(pHead);
}



struct st * create(void)
{
    int a,b,val;
    struct st * pHead;
    struct st * pMid;
    struct st * pNew;
    printf("请输入节点数:");
    scanf("%d",&b);
    pHead=(struct st *)malloc(sizeof(struct st));
    pMid=pHead;
    pMid->pNext =NULL;
    for(a=1;a<=b;a++)
    {
        pNew=(struct st *)malloc(sizeof(struct st));
        printf("请输入第%d个节点的数据:",a);
        scanf("%d",&val);
        pNew->a =val;
        pMid->pNext =pNew;
        pMid=pNew;
        pMid->pNext =NULL;

    }
    printf("数据输入完毕\n");
    return pHead;
}


void show(struct st * pHead)
{
    struct st * pMid;
    if (pHead->pNext ==NULL)
    {
            printf( "链表为空");
            exit(-1);
    }
    else
    {
        pMid=pHead->pNext ;
        while(pMid->pNext !=NULL)
        {
            printf("%d ",pMid->a );
            pMid=pMid->pNext ;
        }
        printf("\n输出完毕");
    }
}

最后只能输入第一个有效节点的值  不知道哪里错了  求教

为什么是pMid!=NULL  而不是 pMid->pNext!=NULL呢？